UC-NRLF 


Mb    27M 


WIRELESS 
TELEGRAPH 


BY  ELMER  E.BUCHE 


Practical  \Vireless  Telegraphy 


A  COMPLETE  TEXT  BOOK 

for  STUDENTS    of 

RADIO  COMMUNICATION 

BY      ' 

ELMER  E.  BUCHER 

Instructing  Engineer 

Marconi  Wireless  Telegraph  Co.  of  America 
Member  Institute  of  Radio  Engineers 


Fully  Illustrated 


WIRELESS  PRESS,  INC 

42  BROAD  STREET,  NEW  YORK 


Ubrary 


COPYRIGHT,  1917 

BY 
WIRELESS  PRESS.  INC. 


S.  L.  PARSONS  &  CO.,  INC.,  Printers,  45  Rose  St.,  New  York. 


AUTHOR'S  NOTE 

In  preparing  this  volume,  the  author  has  endeavored  to  give  the  non-technical  student  and 
the  practical  telegraphist  an  understanding  of  the  functioning  of  present  day  commercial 
wireless  telegraph  apparatus,  and  he  has  varied  the  usual  procedure  followed  in  text  books 
by  covering  first  in  a  general  way  the  fundamentals  of  electricity,  electromagnetic  induction, 
the  dynamo,  the  motor,  the  motor  generator,  storage  batteries,  charging  panels,  etc.,  a 
knowledge  of  which  is  quite  as  essential  to  the  practical  wireless  worker  as  the  more  com- 
plicated phenomena  of  radio-frequency  circuits. 

It  was  not  possible  in  the  space  available  to  treat  the  elements  of  electricity  and 
magnetism  in  detail,  but  an  effort  was  made  to  cover  some  of  the  more  important  prin- 
ciples to  prepare  the  student  to  understand  the  functioning  of  radio  telegraph  apparatus. 

As  in  the  case  of  the  ordinary  electrician  working  in  the  more  common  branches  of 
electrical  practice,  one  of  the  first  essentials  in  training  a  wireless  telegraphist  in  the 
practical  operation  of  a  radio  set  is  to  instil  in  his  mind  a  thorough  understanding  of 
electrical  circuits,  i.  e.,  the  wiring  of  electrical  apparatus,  for  only  as  this  all  important 
knowledge  is  assimilated  is  the  learner  qualified  to  take  charge  of  a  commercial  wireless 
station ;  hence,  this  book  is  largely  devoted  to  describing  the  circuits  of  practical 
radio  sets  together  with  a  simple  explanation  of  the  basic  principles  of  the  apparatus 
associated  therewith. 

No  attempt  therefore  has  been  made  to  treat  the  subject  with  rigid  scientific  accuracy  or 
completeness.  The  idea  has  been  rather  to  show  the  student  what  the  apparatus  consists  of 
and  how  it  is  manipulated.  Only  general  notions  of  how  and  why  it  operates  are  presented 
and  neither  completeness  of  treatment  nor  rigidly  scientific  as  distinguished  from  popular  and 
non-technical  use  of  terms  have  been  attempted. 

In  selecting  the  apparatus  to  be  described  the  author  has  chosen  that  which  is  most  widely 
in  commercial  use  and  such  other  apparatus  as  is  of  general  interest.  In  line  with  this  policy, 
systems  using  radio  frequency  alternative  and  direct  current  arc  transmitters  have  been  treat- 
ed and  chapters  on  undamped  oscillation  receivers  and  Marconi  transoceanic  wireless  tele- 
graph apparatus  have  been  included. 

The  attention  of  prospective  wireless  operators  is  directed  to  the  series  of  questions 
in  the  Appendix  (Section  F),  which  bear  particularly  on  the  salient  points  of  a  practical 
operator's  course  and  which  were  prepared  as  a  guide  for  the  beginner  to  qualify  him  to 
take  the  examination  for  a  Government  license  certificate. 

The  student  who  has  knowledge  of  electrical  circuits  and  requires  instruction  only  in 
the  details  of  commercial  wireless  apparatus  is  advised  to  read  Chapters  Four  to  Twelve 
inclusive,  but  those  who  only  require  a  working  knowledge  of  the  ship  apparatus  used  in  the 
American  Marconi  Company's  service  are  directed  to  Chapters  Nine  and  Twelve. 

One  of  the  first  questions  often  asked  by  a  beginner  who  has  had  no  previous  elec- 
trical training  or  experience  is  "What  is  the  object  of  the  study  of  the  elements  of  elec- 
tricity and  magnetism"  or  "Why  is  such  instruction  required  previous  to  taking  up  the 
subject  of  wireless  apparatus  proper?" 

To  this  it  may  be  answered  that  the  functioning  of  wireless  telegraph  apparatus  is  based 
upon  fundamental  electrical  and  magnetic  principles  and  consequently,  when  the  simple 
laws  of  the  magnet  and  electrical  currents  are.  thoroughly  understood  the  ground  is  at 
least  two-thirds  gone  over.  The  primary  object  of  the  elementary  work  is  to  prepare  the 
student  step  by  step  to  understand  the  apparatus  for  the  production  of  radio-frequent 
currents — the  currents  of  extremely  high  frequency  by  which  the  electric  waves  of  wire- 
less telegraphy  are  set  into  motion.  The  second  object  is  to  explain  and  describe  the 
apparatus  by  which  the  energy  of  these  currents  can  be  radiated  in  the  form  of  electric 
waves  and  detected  at  a  distant  receiving  station,  and  to  explain  the  apparatus  by  which 
such  currents  are  finally  made  audible  in  a  telephone  receiver  or  some  sort  of  telegraphic 
recording  apparatus. 

The  author  desires  to  acknowledge  his  indebtedness  to  the  Marconi  Wireless  Telegraph  Co. 
of  America,  the  Crocker-Wheeler  Manufacturing  Co.  and  the  Electric  Storage  Battery  Com- 
pany for  the  loan  of  photographs,  cuts,  blue  prints,  wiring  diagrams  and  literature  which  have 
greatly  assisted  in  the  preparation  of  this  work.  He  has  also  freely  consulted  the  columns  of 
the  Wireless  Age  and  the  Proceedings  of  the  Institute  of  Radio  Engineers. 

E.  E.  B. 

New  Yor4c,  June,   1917.  ' 

oooUlo 


CONTENTS 

PART  I. 

MAGNETISM. 

THE   MAGNETIC   CIRCUIT. 

J>-jO-£ 

1.    Natural    Magnet.      2.    Flux.      3.    Polarity.      4.    Magnetic    Induction.      5.    Permanent   and   Temporary 

Magnets.      6.   Laws  of   Magnetic   Poles.      7.    Magnetic    Circuit 1 

PART  II. 
THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE. 

ELECTRIC   CURRENT   AND    CIRCUITS. 

8.  Electrical  Current.  9.  Classification  of  Currents.  10.  Electromotive  Force.  11.  Conductors  and 
Insulators.  12.  Production  of  Electromotive  Force.  13.  Electricity  by  Friction  (Static  Elec- 
tricity). 14.  Electricity  by  Chemical  Action  (Primary  or  Secondary  Batteries).  15.  Secondary 
Cell.  16.  Current  Strength  and  Quantity.  17.  Electrical  Resistance.  18.  Grouping  of  Electrical 
Cells.  19.  Ohm's  Law  and  Practical  Application.  20.  Divided  Circuits.  21.  Electrical  Work. 
22.  Electrical  Horsepower.  23.  Definition  of  Electrical  Units.  24.  Current  Output  and  Voltage 
of  Various  Current  Sources 4 

PART  III. 
ELECTROMAGNETIC   INDUCTION. 

THE  DYNAMO— THE  FLOW  OF  ALTERNATING  CURRENT. 

25.  Electromagnetism.  26.  Magnetic  Field  About  Two  Parallel  Conductors.  27.  The  Solenoid. 
28.  Induced  Currents.  29.  Mutual  Induction.  30.  Self-induction.  31.  Value  of  Induced 
E.  M.  F.  32.  The  Dynamo.  33.  Determination  of  Frequency.  34.  Strength  of  Magnetic  Field. 
35.  Diagram  of  an  Alternating  Current  Dynamo.  36.  Direct  Current  Dynamo.  37.  Shunt, 
Series  and  Compound  Wound  Dynamos.  38.  The  Electric  Motor.  39.  The  Effect  of  Counter 
Electromotor  Force.  40.  Motor  with  Differential  Field  Winding.  41.  Dynamo  and  Motor 
Armatures.  42.  Development  of  Armature  Windings.  43.  The  Alternating  Current  Transformer. 
44.  Electrostatic  Capacity.  45.  Reactance  and  Impedance.  46.  Capacity  Reactance.  47.  Lag 
and  Lead  of  Alternating  Current.  48.  Effective  Value  of  Alternating  E.  M.  F.  and  Current. 
49.  Measuring  Instruments  or  Electric  Meters.  50.  Induction  Coil.  51.  Practical  Electric 
Circuits 16 

PART  IV. 
MOTOR    GENERATORS. 

HAND    AND   AUTOMATIC  MOTOR   STARTERS. 

52.  The  Motor  Generator.  53.  Field  Rheostats.  54.  Dynamotor  and  Rotary  Converter.  55.  The 
Motor  Starter.  56.  Automatic  Motor  Starters.  57.  Protective  Condensers.  58.  Care  of  the 
Motor  Generator.  59.  How  to  Remove  Motor  Generator  Armature » 51 

/ 

PART  V. 
STORAGE  BATTERIES  AND  CHARGING  CIRCUITS. 

60.  The  Necessity  for  a  Storage  Battery  in  a  Radio  Installation.  61.  General  Construction  and 
Action.  62.  The  Charging  Process.  63.  The  Fundamental  Actions  of  a  Storage  Cell.  64.  The 
Electrolyte.  65.  The  Hydrometer.  66.  How  the  Capacity  of  a  Storage  Cell  is  Rated.  67.  Funda- 
mental Facts  Concerning  the  Storage  Cell.  68.  How  to  Charge  a  Storage  Cell.  69.  How  to 
Determine  the  Value  of  the  Charging  Resistance.  70.  Lamp  Bank  Resistance.  71.  The  Use  of 
the  Ammeter  and  the  Underload  Circuit  Breaker.  72.  The  Ampere  Hour  Meter.  73.  Over- 
charge. 74.  How  to  Charge  a  Battery  When  the  Voltage  Exceeds  That  of  the  Charging  Gen- 
erator. 75.  How  to  Determine  the  Polarity  of  the  Charging  Generator.  76.  Determination  of  the 
State  of  Charge  and  Discharge  of  a  Battery.  77.  Keeping  the  Level  of  the  Electrolyte.  78.  Pro- 
tecting the  Cells  from  Acid  Spray.  79.  General  Instructions  for  the  Portable  Chloride  Type  of 
Accumulators.  80.  General  Operating  Instructions  for  the  Exide  Cell.  81.  The  Edison  Storage 
Battery.  82.  The  Charge  and  Discharge  of  the  Edison  Cell 67 


vi  PRACTICAL   WIRELESS   TELEGRAPHY. 

PART  VI. 
THE  RADIO  TRANSMITTER. 

CONDENSERS— OSCILLATION      GENERATORS— RADIATION      OF      ELECTRICAL      WAVES- 
DAMPING  OF  OSCILLATIONS. 

Page 

83.  Methods  of  Generating  Radio  Frequency  Current.  84.  The  Condenser.  85.  Connections  for  Con- 
densers. 86.  How  to  Place  a  Charge  in  a  Condenser.  87.  Analysis  of  a  Spark  Discharge. 
88.  Effect  of  Resistance  on  Oscillations.  89.  Electrical  Resonance.  90.  The  Open  Circuit 
Oscillator.  91.  The  Length  of  the  Electric  Wave.  92.  The  Determination  of  Wave  Length 
From  the  Inductance  and  Capacity.  93.  Logarithmic  Decrement  of  the  Oscillations.  94. 
Methods  of  Exciting  Oscillations  in  an  Aerial.  95.  The  Reaction  of  Coupled  Circuits.  96.  The 
Standard  Waves  of  Commercial  Wireless  Telegraphy.  97.  Fundamental  Circuit  of  a  Complete 
Radio  Transmitter.  98.  Simple  Explanation  of  the  Circuits.  99.  Numerical  Values  for  a 
Standard  Radio  Set 80 


PART  VII. 
APPLIANCES    FOR   A    RADIO    TRANSMITTER. 

SPARK    DISCHARGERS— OSCILLATION    TRANSFORMERS— CONDENSERS— TRANSFORMERS. 

100.  In  General.  101.  Spark  Dischargers  for  Radio  Telegraphy.  102.  Adjustment  of  the  Spark  Note. 
103.  Oscillation  Transformers.  104.  Aerial  Tuning  Inductance.  105.  The  Short  Wave  Condenser. 
106.  High  Potential  Condensers.  107.  High  Frequency  "Choking"  Coils.  108.  High  Voltage 
Transformers.  109.  Reactance  Regulators.  110.  Aerial  Changeover  Switch.  111.  Transmitting 
Keys  101 


PART  VIII. 
AERIALS  OR  ANTENNAE. 

112.  Function  of  the  Aerial.  113.  Determination  of  the  Wave  Length  From  the  Dimensions  of 
an  Aerial.  114.  Fundamental  Considerations.  115.  Various  Types  of  Aerials.  116.  Directional 
Aerials.  117.  Standard  Marconi  Aerial.  118.  The  Deck  Insulator.  119.  Installation  of  the 
Aerial.  120.  Earth  Connection.  121.  Radiation.  122.  Antenna  Decrement.  123.  Transmis- 
sion Range • 116 


PART  IX. 
RECEIVING  CIRCUITS,  DETECTORS  AND  TUNING  APPARATUS. 

STANDARD    MARCONI    RECEIVING    SETS. 

,24.  In  General.  125.  The  Problem.  126.  Simple  Receiver.  127.  The  Inductively  Coupled  Re- 
ceiver. 128.  Other  Methods  of  Coupling.  129.  The  Carborundum  Detector  and  Tuning  Cir- 
cuits. 130.  Adjustment  of  the  Inductively  Coupled  Tuner.  131.  The  Action  of  the  Car- 
borundum Crystal.  132.  Adjustment  of  Crystal  Detectors.  133.  Detector  Holders.  134.  Classi- 
fication of  the  Receiving  Detectors.  135.  Fleming  Valve  Detector  and  Tuning  Circuits.  136. 
Marconi  Type  107-A  Tuner.  137.  Marconi  Magnetic  Detector  and  the  Multiple  Tuner  Circuits 
(English  Marconi  Company).  138.  The  Marconi  Type  106  Receiving  Tuner.  139.  Marconi 
Receiving  Tuner  Type  101  (American  Marconi  Company).  140.  The  Marconi  Universal  Re- 
ceiving Set  (English  Marconi  Company).  141.  Electrolytic  Detector.  142.  The  Three  Element 
Valve  Detector.  143.  A  Repeater  Vacuum  Valve  Circuit.  144.  The  Vacuum  Valve  Amplifier. 
145.  Amplification  of  Radio  Frequencies.  146.  The  Effects  of  Distributed  Capacity.  147.  The 
"End  Turns"  of  a  Receiving  Tuner  and  End  Turn  Switches.  148.  The  Variation  of  a  Radio 
Frequency  Inductance.  149.  Buzzer  Excitation  Systems.  150.  Receiving  Telephones.  151.  Mi- 
crophonic  Relays  or  Sound  Intensifiers.  152.  Brown  Amplifying  Relay.  153.  Atmospheric 
Electricity.  154.  The  Marconi  Balanced  Crystal  Receiver  (English  Marconi  Company).  155. 
Type  I  Aerial  Changeover  Switch.  156.  Marconi  Type  112  Receiving  Tuner.  156A.  General 
Advice  for  the  Manipulation  of  a  Receiving  Tuner 129 

PART  X. 
AUXILIARY    APPARATUS    OR   EMERGENCY   TRANSMITTERS. 

!57.  Statute    Requirements.      158.  Tuned    Coil    Set.      159.  The    Electric    Storage    Battery    Company's 

Accumulators    and    Charging    Panel 179 


CONTENTS.  vii 

PART  XI. 
PRACTICAL  RADIO  MEASUREMENTS. 

MEASUREMENT  OF  WAVE   LENGTH— DECREMENT  -  CALIBRATION— TRANSMITTING   AND 

RECEIVING  APPARATUS. 

Page 

160.  The  Importance  of  Electrical  Resonance.  161.  Indicators  of  Resonance.  162.  Uses  of  the 
Wavemeter.  163.  Simple  Use  of  the  Wavemeter.  164.  General  Instructions  for  Tuning  a 
Radio  Transmitter.  165.  Tuning  by  the  Hot  Wire  Ammeter.  166.  Tuning  the  2  K.  W.  500 
Cycle  Panel  Transmitter.  167.  Determination  of  Coupling.  168.  Plotting  of  Resonance  Curves. 
169.  Measurement  of  the  Logarithmic  Decrement  of  Damping.  170.  Calculation  of  the  Decre- 
ment of  the  Wavemeter  (or  Decremeter).  171.  Wavemeter  as  a  Source  of  High  Frequency 
Oscillations.  _  172.  Calibration  of  the  Secondary  and  Primary  Circuits  of  a  Receiving  Tuner. 
173.  Calibration  of  the  Open  and  Closed  Circuits  Simultaneously.  174.  Measurement  of  the 
Natural  Oscillating  Period  of  a  Coil.  175.  Measurement  of  Electrostatic  Capacity.  176.  Meas- 
urement of  the  Effective  Inductance  of  a  Coil  at  Radio  Frequencies.  177.  Calculation  of  In- 
ductance from  the  Constants  of  the  Coil.  178.  Measurement  of  the  Effective  Inductance  and 
Capacity  of  an  Aerial.  179.  Calibration  of  a  Wavemeter  from  a  Standard.  180.  Measurement 
of  Mutual  Inductance  at  Radio  Frequencies.  181.  Comparative  Measurement  of  the  Strength 
of  Incoming  Signals.  182.  "Tight"  and  "Loose"  Coupling.  183.  Measurement  of  High 
Voltages.  184.  Tuning  and  Adjustment  Record 188 

PART  XII. 
STANDARD  MARINE  SETS  OF  THE  AMERICAN  MARCONI  COMPANY. 

PANEL  TRANSMITTERS— COMPOSITE  TRANSMITTERS. 

185.  Panel  Transmitters.  186.  Details  of  Type  P-4  Panel.  187.  Description  of  Apparatus.  188. 
Complete  Adjustment  of  Type  P-4  Set.  189.  Type  P-5  Panel  Transmitter.  190.  Description 
of  Apparatus.  191.  Complete  Adjustment  of  the  Type  P-5.  Set.  192.  How  to  Remove  the  Arma- 
ture of  the  %  K.  W.  Motor  Generator.  193.  The  1  K.  W.  Non-Synchronous  Discharger 
Transmitter.  194.  Description  of  the  Set.  195.  Installation.  196.  Adjustment  of  the  1  K.  W. 
Set.  197.  Type  "E-2"  One-half  Kilowatt,  120  Cycle  Panel  Transmitter.  198.  Details  of  the 
Circuits  and  Apparatus.  199.  General  Instructions  for  Tuning  and  Adjusting.  200.  Marconi 
2  K.  W.  240  Cycle  Transmitter.  201.  Type  P-9  Y*  K.  W.  Cargo  Transmitting  Set.  202. 
Aerial  Current  and  Reduction  of  Pover.  203.  General  Instructions  for  the  Panel 223 

PART  XIII. 

MARCONI    DIRECTION    FINDER   OR   WIRELESS    COMPASS   AND   ITS 

APPLICATION. 

204.  In  General.  205.  Description  of  Equipment.  206.  The  Direction  Finder  Aerials.  207.  The 
Circuit  Complete.  208.  The  Tuned  Buzzer  Tester.  209.  How  Current  is  Induced  in  the  Looped 
Aerials.  210.  Direction  of  Magnetic  Forces  Within  the  Goniometer.  211.  General  Instructions 
for  Operation  of  the  Direction  Finder.  212.  To  find  the  Direction  of  a  Radio  Station 255 

PART  XIV. 
TRANSMITTERS  OF  UNDAMPED  OSCILLATIONS. 

ARC    GENERATORS— RADIO-FREQUENCY    ALTERNATORS— PLIOTRON    OSCILLATOR. 

213.  In  General.  214.  The  Arc  Generator.  215.  Signalling  with  the  Arc  Transmitter.  216.  The 
Alexanderson  High  Frequency  Alternator.  217.  Goldschmidt  Radioi-'Frequency  Alternator. 
218.  The  Joly  System  for  the  Protection  of  Undamped  Oscillations.  219.  Marconi's  System  for 
the  Production  of  Continuous  Waves.  220.  The  Pliotron  Oscillator 264 

PART  XV. 
RECEIVERS   FOR  UNDAMPED   OSCILLATIONS   OR  CONTINUOUS  WAVES. 

221.  The  Problem.  222.  The  Tikker.  223.  The  Heterodyne  System.  224.  The  Vacuum  Valve  as 
a  Source  of  Radio-Frequency  Oscillations.  225.  Vacuum  Valve  as  a  Combined  Detector, 
Amplifier  and  Beat  Receiver.  226.  Oscillating  Vacuum  Valve  Detector  Circuits  of  the  U.  S. 
Navy.  227.  The  Goldschmidt  Tone  Wheel.  228.  Marconi  System  for  Reception  of  Undamped 
Oscillations , 277 

PART  XVI. 
MARCONI   TRANSOCEANIC    RADIO    TELEGRAPHY. 

229.  Marconi  Development  and  General  Considerations.  230.  Marconi's  Duplex  System.  231.  The 
Balancing  Out  Aerial.  232.  Glace  Bay-Clifden  Stations.  233.  Marconi  Directional  Aerial. 
234.  Marconi  Transoceanic  Stations.  235.  Marconi  Tubular  Masts.  236.  Radio-Frequency 
Circuits  of  the  Damped  Wave  Transmitters.  237.  Other  U.  S.  High  Power  Stations.  238.  Long 
Distance  Receiving  Sets.  239.  Condensed  List  of  High  Power  Stations 288 


Practical  Wireless  Telegraphy 

PART  I. 
MAGNETISM. 

THE  MAGNETIC  CIRCUIT. 

1.  NATURAL  MAGNET.  2.  FLUX.  3.  POLARITY.  4.  MAGNETIC 
INDUCTION.  5.  PERMANENT  AND  TEMPORARY  MAGNETS. 
6.  LAWS  OF  MAGNETIC  POLES.  7.  MAGNETIC  CIRCUIT. 

Because  the  flow  of  an  electrical  current  is  invariably  accompanied  by  a  mag- 
ictic  field,  a  brief  explanation  of  the  phenomena  surrounding  the  simple  bar 
nagnet  will  be  given.  This  is  to  be  followed  in  a  successive  chapter  by  a  descrip- 
ion  of  the  electromagnet. 

1.  Natural  Magnet. — A  substance  which  has  the   property   of  attracting 
>its  of  iron  or  steel  is  called  a  magnet.    Natural  magnets  found  in  various  parts  of 
he  earth  are  known  as  lodestone  and  a  piece  of  lodestone  dipped  into  a  pile  of 
ron  or  steel  filings  exhibits  this  property  of  attraction  to  a  considerable  degree. 

If  a  bar  of  hard  steel  be  rubbed  with  a  piece  of  lodestone  the  steel  is  found  to 
)e  magnetized  and  is  then  known  as  an  artificial  magnet.  If  the  same  bar  is 
lipped  into  a  pile  of  iron  filings,  the  majority  of  the  filings  cling  to  the  tips  of  the 
>ar,  there  being  no  tendency  towards  attraction  at  the  center.  Since  the  strongest 
nagnetism  exists  at  the  ends  of  the  bar,  these  ends  are  known  as  the  poles  of  the 
nagnet. 

2.  Flux. — If  a  piece  of  paper,  over  which  iron  filings  have  been  sprinkled, 
s  placed  above  and  parallel  to  a  bar  magnet,  the  filings  will  arrange  themselves 

^  _^  into  a  series  of  well  defined  lines  as  in 

^""  ~^^N  Fig.  1.    These  may  be  said  to  show  the 

/         ,--—•• -^         NN  general  direction  of  the  magnetic  force. 

\        I      /'  ^NN      \         /       These  lines  indicate  that  the  space  about 

\      *     /          ^ -•*--»„         \     f.      *'       the  poles  of  a  magnet  is  in  a  state  of 
\  \  \       '  /      /  /     '       .    stress  or  strain,  and  therefore,  they  are 

called  the  magnetic   lines  of  force   or 
simply  lines  of  force.     The  space  sub- 


'  \     \  "\    N  jected  to  this  strain  is  called  the  mag- 

^__^ -'        i     \     \        netic  field  and  the  total  lines  of  force 

1        \      crossing   a    given    space    or    field    are 


/ 


^-~^  —  -  /  termed  the  magnetic  flux. 


3.  Polarity.  —  A     magnetic     needle 
Fig.  i-Fieidf  a  Simple  Bar  Magnet.  suspended  or  pivoted  as  in  a  compass 

and  left  to  swing  freely  will,  as  is  well 

known,  point  in  the  direction  of  the  north  magnetic  pole.  The  end  which  points 
in  that  direction  is  known  as  the  north  pole  of  the  magnet  and  the  opposite  end, 
the  south  pole. 


2  PRACTICAL  ; WIRELESS  TELEGRAPHY 

4.  Magnetic  Induction.— A'  piqce  of  soft  iron  placed  in  the  magnetic  field 
of  another  niftgixct, ".  fcrecoja'Jes  .temporarily  magnetized  and  will  have  two  unlike 
poles.     Magnetism  H'htis  ;  indtice(i  in  a  piece  of  soft  iron  is  said  to  be  due  to 
magnetic  induction.     If,  for  example,  the  north  pole  of  a  steel  bar  magnet  be 
placed  near  to  a  bar  of  soft  iron,  the  end  of  the  iron  bar  nearest  to  the  magnet 
will  exhibit  south  magnetism  and  the  opposite  end  north  magnetism.     It  should 
be  understood  that  magnetism  can  be  induced  in  the  iron  bar  whether  in  direct 
contact  with  the  inducing  magnet  or  slightly  separated  from  it  but  when  the 
exciting  magnet  is  removed,  the  induced  magnetism  will  practically  disappear. 

5.  Permanent  and  Temporary  Magnets. — Because  a  bar  of  soft  iron  re- 
tains its  magnetism  only  while  under  the  influence  of  a  given  magnetizing  force 
it  is  called  a  temporary  magnet.    On  the  other  hand  a  piece  of  steel,  when  once 
magnetized,   retains  its  magnetism  permanently,  and  thereafter  is  known  as  a 
permanent  magnet. 

LINES  IN  OPPOSITE  DIRECTION 
REPULSION 


LINES  IN  SAME  D1RECT10H 
ATTRACTION 


m 

.-•>)}  rcc 


Fig.  2 — Diagram  Showing  the  Attraction  and  Repulsion  of  Magnetic  Fields. 

The  power  of  steel  to  resist  magnetization  and  once  in  this  condition  to  resist 
demagnetization  is  termed  its  retentivity.  Steel  possesses  greater  reten- 
tivity  than  iron  because,  as  previously  mentioned,  soft  iron  becomes  saturated 
with  magnetism  very  quickly  and  loses  it  almost  immediately  when  the  inducing 
magnetic  field  is  removed. 

The  capability  of  any  substance  for  conducting  magnetic  lines  of  force  is 
termed  its  permeability.  Iron,  for  instance,  possesses  much  greater  permeability 


MAGNETISM.  3 

• 

than  steel  and  steel  possesses  greater  permeability  than  air.  This  means 
that  if  the  circuit  for  the  magnetic  lines  of  force  from  pole  to  pole  of  a  magnet 
is  completed  through  an  iron  core,  a  greater  number  of  lines  of  force  will  pass 
than  if  the  circuit  were  completed  through  a  piece  of  steel  or  through  air. 

6.  Laws  of  Magnetic  Poles. — If  two  bar  magnets  are  suspended  by  a  cord 
as  in  Fig.  2,  and  the  north  pole  of  one  brought  near  to  the  north  pole  of  the 
other,  they  will  be  found  to  repel.     On  the  other  hand,  if  the  south  pole  of  a  bar 
magnet  is  brought  near  to  the  north  pole  of  another  magnet,  they  are  found  to 
attract  one  another.     The  foregoing  actions  may  be  summed  up  by  the  funda- 
mental law :    Like  magnetic  poles  repel,  unlike  magnetic  poles  attract. 

A  variation  of  this  law  is  encountered  when  a  very  strong  south  pole,  let  us 
say,  is  placed  near  a  weak  south  pole.  The  stronger  magnet  will  attract  the  weaker 
one  because  of  its  over-powering  field.  Similar  effects  are  observed  between  two 
north  poles  of  dissimilar  strength. 

7.  Magnetic  Circuit. — Each  line  of  force  of  a  magnet   (as  in  Fig.   1)   is 
assumed  to  pass  from  the  south  pole  to  the  north  pole,  through  the  bar  and 
from  the  north  to  the  south  pole  outside  the  bar.    This  is  said  to  be  the  direction 
of  the  lines  of  force  and  the  path  they  take  is  called  the  magnetic  circuit.    Such  a 
circuit  is  usually  made  up  of  magnetic  material  like  iron  or  steel  but  in  com- 
mercial apparatus  very  often  one  or  more  air  gaps  are  included  in  the  path  of  the 
flux. 

If  a  magnetic  substance  such  as  a  bar  of  iron  is  suspended  free  to  move  in  a  magnetic  field, 
it  ivill  tend  to  turn  and  lie  parallel  with  the  Held,  or,  as  is  more  often  said,  ^vill  take  such  a 
position  as  to  accommodate  through  itself  the  greatest  number  of  lines  of  force.  On  the  other 
hand,  if  a  permanent  magnet  is  suspended  free  to  move  in  a  magnetic  field  of  definite  direction 
(such  as  suspending  a  bar  magnet  above  a  stationary  magnet}  it  will  tend  to  take  a  parallel 
position  with  the  field  but  in  a  particular  direction,  that  is,  its  internal  lines  of  force  will  be 
in  the  same  direction  as  those  of  the  field. 

Advantage  of  this  fundamental  principle  is  taken  in  the  design  of  many  electromagnetic 
devices  and  in  electrical  measuring  instruments  to  be  described  later  on. 

Powerful  magnetic  fields  may  be  created  by  an  electric  current.  Mag- 
netism so  produced  is  known  as  electromagnetism.  The  great  advantage  of  the 
electromagnet  is  the  fact  that  the  strength  of  the  magnetic  field  can  always  be 
controlled,  whereas  the  field  of  the  permanent  magnet  is  more  or  less  of  fixed 
strength.  Electromagnetism  will  be  taken  up  in  its  proper  order  in  a  following 
chapter. 


PART  II. 

THE   PRODUCTION    OF    ELECTROMOTIVE 

FORCE. 

ELECTRIC    CURRENT  AND   CIRCUITS. 

8.  ELECTRICAL  CURRENT.  9.  CLASSIFICATION  OF  CURRENTS. 
10.  ELECTROMOTIVE  FORCE.  11.  CONDUCTORS  AND  INSULA- 
TORS. 12.  PRODUCTION  OF  ELECTROMOTIVE  FORCE.  13.  ELEC- 
TRICITY BY  FRICTION  (STATIC  ELECTRICITY).  14.  ELECTRICITY 
BY  CHEMICAL  ACTION  (PRIMARY  OR  SECONDARY  BATTERIES). 
15.  SECONDARY  CELL.  16.  CURRENT  STRENGTH  AND  QUANTITY. 
17.  ELECTRICAL  RESISTANCE.  18.  GROUPING  OF  ELECTRICAL 
CELLS.  19.  OHM'S  LAW  AND  PRACTICAL  APPLICATION. 
20.  DIVIDED  CIRCUITS.  21.  ELECTRICAL  WORK.  22.  ELECTRICAL 
HORSEPOWER.  23.  DEFINITION  OF  ELECTRICAL  UNITS.  24.  CUR- 
RENT OUTPUT  AND  VOLTAGE  OF  VARIOUS  CURRENT  SOURCES. 

8.  Electric  Current. — When  we  speak  of  a  current  of  electricity  as  flowing 
through   a  wire  or  circuit  we  simply  express  in   a  convenient   way  certain 
phenomena  associated  therewith.    We  do  not,  in  fact,  know  what  actually  trans- 
pires in  the  transfer  of  electricity  from  point  to  point  in  a  conductor.   Electricians 
generally  agree  that  a  so-called  "current"  of  electricity  flows  in  a  definite  direc- 
tion throughout  a  given  circuit,  but  there  is  no  direct  evidence  at  hand  to  prove 
the  actual  existence  of  a  "current",  in  the  commonly  accepted  meaning  of  the 
word.    The  term,  however,  is  universally  adopted  to  designate  the  flow  of  elec- 
tricity from  point  to  point  in  an  electrical  circuit. 

9.  Classification  of  Currents. — Electrical  currents  are  called   direct  if  they 
flow  in  one  direction  throughout  a  given  circuit,  and  alternating  if  they  con- 
tinually reverse,  flowing  first  in  one  direction  and  then  in  the  other. 

A  primary  current  is  said  to  be  one  which  flows  directly  from  a  generating 
source.  A  secondary  current  is  one  induced  by  a  primary  current  acting  in- 
ductively on  a  circuit  having  no  direct  connection  with  the  primary  circuit. 
A  current  is  said  to  be  of  low  tension  when  its  pressure  or  voltage  is  rela- 
tively low,  and  conversely,  it  is  said  to  be  of  high  tension  when  its  pressure 
or  voltage  is  relatively  high. 

10.  Electromotive  Force. — In  order  to  produce  a  steady  electrical  current, 
two  conditions   are   necessary.      There   must  be   a   steadily   maintained   electric 
pressure  known  as  electromotive  force  and  a  suitable  conducting  path  to  pass 
the  current. 

11.  Conductors  and  Insulators. — A  metallic  circuit  in  which  a  current  of 
electricity  flows   with   little   opposition   is   said   to   be   a   conductor;   one   which 
offers  considerable  resistance  is  known  as  a  partial  conductor,  but  a  substance 
which  completely  impedes  the  flow  of  current  is  termed  an  insulator.     It  should 
be  understood  at  the  beginning  that  these  terms  are  purely  relative  for  an  abso- 
lute insulator  or  a  perfect  conductor  does  not  exist. 

The  best  conductors  of  an  electric  current  among  the  common  metals,  in  order  of 
their  increasing  resistance,  are  silver,  copper,  gold,  aluminum,  zinc,  iron,  platinum  and 
nickel. 

Examples  of  insulators  given  in  order  of  their  increasing  value  are  dry  air,  shellac, 


THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE. 


paraffine,   amber,   resin,   sulphur,   wax,   glass,   mica,    ebonite,   india   rubber,   silk,   paper 
and  oils. 

12.  Production   of   Electromotive    Force. — To   produce    an    electromotive 
force,  it  is  necessary  first  to  create  a  difference  in  potential  or  difference  in  electric 
pressure  between  two  bodies  or  two  points  in  the  same  body. 

An  electromotive  force  can  be  produced  by  various  methods,  for  example : 

(1)  By  friction  (static  machine); 

(2)  By  chemical  action  (primary  and  secondary  batteries) ; 

(3)  By  mechanical  motion  (dynamos  or  generators) ; 

(4)  By  thermal  action  (thermo  junction). 

In  the  following  chapters  these  four  methods  will  be  considered  consecutively 
and  in  detail. 

Electromotive  force  is  denoted  by  the  unit  termed  the  volt.  The  term 
pressure  and  voltage  are  used  to  express  difference  of  potential  or  electromotive 
force  (abbreviated  E.  M.  E.)  as  well. 

13.  Electricity  by  Friction  (Static  Electricity).— When  a  piece  of  amber 
is  rubbed  with  silk  the  amber  is  said  to  be  electrified,  and  the  presence  of  this 
electrification  can  be  detected  by  holding  the  amber  near  to  small  bits  of  paper. 
The  paper  will  be  attracted  to  the  amber.     The  silk  is  also  in  a  state  of  electri- 
fication and  if  it  is  held  near  to  another  piece,  similarly  electrified,  it  will  be 
repelled.     Likewise  two  pieces  of  electrified  amber  will  repel  one  another,  and 
if  the  amber  is  held  near  the  silk,  the  silk  will  be  attracted  to  it. 

This  action  of  attraction  and  repulsion  is  said  to  be  due  to  electric  charges 
residing  on  these  elements.  The  amber  is  said  to  possess  positive  (  +  )  electri- 
fication and  the  silk  negative  ( — )  electrification.  The  electric  charges  are 
said  to  be  caused  by  friction  and  are  known  as  static  electricity,  meaning  electric 
charges  at  rest  or  stationary. 

It  is  to  be  noted  that  if  a  body  containing  a  positive  charge  is  brought 
in  contact  with  one  containing  a  negative  charge,  both  charges  being  of  equal 
intensity,  they  will  neutralize  and  disappear ;  the  bodies  are  then  to  be  discharged. 
Again  if  two  charged  bodies  are  joined  by  an  electric  conductor,  all  signs  of 
electrification  will  disappear  and  there  will  pass  through  the  conductor  a  momen- 
tary electric  current. 

There  are  other  elements  which  when 
rubbed  together  will  produce  static  charges 
of  electricity,  but  the  foregoing  example 
is  sufficient  to  illustrate  the  method. 
Machines  for  the  production  of  electro- 
motive force  by  friction  are  known  as 
static  or  frictional  machines  but  since  they 
bear  no  particular  relation  to  the  principles 
involved  in  the  functioning  of  wireless  tele- 
graph apparatus,  a  description  will  not  be 
given. 

14.  Electricity  by  Chemical  Action 
(Primary  or  Secondary  Batteries). — 
A  convenient  and  practical  appar- 
atus for  setting  up  a  steady  electro- 
motive force  is  the  electrochemical 
cell  which  consists  of  two  dissimilar 
elements,  in  other  words,  two  un- 
like metals  immersed  into  a  dilute 
acid  or  alkali  solution. 

A  simple  cell,  for  example,  consists  of 
strips  of  zinc  and  carbon  immersed  in  a 
conducting  solution  of  sal  ammoniac  (am- 
monium chloride)  as  in  Fig.  3.  If  the  Fig.  3— Simple  Electric  Cell. 


6  PRACTICAL   WIRELESS    TELEGRAPHY 

exposed  terminals  of  these  plates  are  joined  by  a  metallic  conductor,  the  cell  is  capable  of 
supplying  a  continuous  flow  of  electricity  through  the  wire.  It  is  observed  as  the  current 
flows  that  the  zinc  strip  wastes  away,  in  fact,  the  consumption  of  the  zinc  furnishes  the  electro- 
motive force  necessary  to  drive  the  current  through  the  cell  and  through  the  external  circuit. 
The  chemical  changes  within  the  cell,  consisting  of  copper  and  zinc  strips  immersed  in  a 
dilute  solution  of  sulphuric  acid  may  be  briefly  described  as  follows:  When  the  copper 
and  zinc  strips  are  connected  together  by  a  metallic  circuit  and  the  current  begins  to 
flow,  the  sulphuric  acid  attacks  the  surface  of  the  zinc  plate  and  forms  a  compound  known 
as  sulphate  of  zinc.  During  the  formation  of  this  sulphate  some  of  the  hydrogen  contained 
in  the  sulphuric  acid  is  liberated  in  the  form  of  bubbles  which  immediately  appear  on  the 
copper  plate.  Some  of  these  bubbles  rise  to  the  surface  of  the  liquid  and  escape  into  sur- 
rounding air,  but  others  cling  to  the  copper  plate  which  gradually  becomes  covered  with  a 
film  of  hydrogen.  Since  hydrogen  is  a  non-conductor  of  electricity,  the  amount  of  surface 
of  the  copper  plate  in  contact  with  the  battery  solution  gradually  decreases  as  the  accumula- 
tion of  hydrogen  gas  increases,  and  accordingly  the  current  output  of  the  cell  diminishes. 
In  electrician's  parlance  the  cell  is  now  said  to  be  "run  down."  Part  of.  this  reduction  of 
current  is  due  to  the  fact  that  hydrogen  tends  to  set  up  a  current  within  the.  cell  in  the 
opposite  direction  to  the  normal  flow  as  well  as  cover  the  copper  plate.  A  cell  in  this 
condition  is  said  to  be  polarized,  and  various  chemical  and  mechanical  means  have  been 
devised  to  prevent  the  hydrogen  bubbles  clinging  to  the  copper  plate. 

An  electroscope  (a  device  for  determining  the  presence  and  nature  of  electric 
charges)  indicates  a  strongly  negative  charge  at  the  exposed  end  of  the  zinc 
element.  The  zinc  plate  is  therefore  known  as  the  negative  ( — )  pole  of  the 
cell,  and  the  carbon  or  copper  terminal,  the  positive  (  +  )  pole  of  the  cell. 

We  learn  from  this  that  the  action  of  the  battery  solution  upon  one  plate  more 
than  on  the  other  tends  to  keep  the  plates  in  a  continuous  state  of  electrification, 
the  stronger  manifestation  being  exhibited  at  the  exposed  end  of  the  zinc  plate 
and  it  is  this  difference  in  pressure  which  causes  the  current  to  flow  round  the 
external  circuit. 

The  direction  of  the  current  inside  the  cell  will  be  from  the  zinc  plate  through 
the  solution  to  the  carbon  plate  and  outside  the  cell  from  the  carbon  plate  through 
a  metallic  conductor  to  the  zinc  plate. 

The  conducting  fluid  in  which  the  elements  of  the  electric  cell  are  immersed 
is  known  as  the  electrolyte  or  the  exciting  fluid.  The  plates  and  the  metallic 
terminals  attached  thereto  are  termed  the  poles  or  electrodes  of  the  cell.  A 
number  of  cells  connected  together  are  known  as  a  battery. 

The  type  of  cell  just  described  is  called  a  primary  cell  to  distinguish  it  from 
a  storage  or  secondary  cell  which  will  be  described  in  detail  further  on. 

It  has  been  mentioned  that  the  electromotive  force  or  corresponding  flow  of  current  pro- 
duced by  the  electrochemical  cell  is  caused  by  two  dissimilar  elements.  The  list  of  metals 
given  below  are  arranged  in  such  order  that  any  single  element  will  be  the  negative  pole  of 
the  battery  when  used  with  the  metal  next  below  it  on  the  list,  and  the  positive  pole  when 
used  with  the  element  next  above  it. 

( — )   Sodium  Iron  * 

Magnesium  Copper 

Zinc  Silver 

Tin  Gold 

Cadmium  Platinum 

Lead  Carbon       (+) 

Referring  to  this  list,  although  there  will  be  a  difference  of  potential,  and  consequently  a  flow 
of  current  between  carbon  and  copper  if  joined  together  by  a  wire  and  immersed  in  a  battery 
solution,  there  will  be  a  very  much  greater  electromotive  force  if  carbon  and  zinc  are  employed. 
15.  Secondary  Cell. — A  simple  secondary  cell  popularly  known  as  a 
"storage  battery"  consists  of  two  or  more  plates  of  lead  placed  in  a  dilute 
solution  of  sulphuric  acid  as  in  Fig.  4.  One  of  the  plates  in  this  diagram  is 
connected  to  the  positive  terminal  of  two  primary  cells  (connected  in  series) 
and  the  other  plate  to  the  negative  terminal.  When  current  flows  from 
the  primary  battery  for  some  time  through  the  solution  from  plate  to  plate  as  in 
Fig.  5  and  afterwards  the  wires  from  the  primary  battery  are  disconnected  and 

FOOTNOTE:— Cells  of  various  types  are  described  in  books  on  elementary  electricity  to  which  the  reader  is 
referred  for  a  more  detailed  description. 


THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE. 


ANODE 


LEAD  mp^  m 


SECONDARY 
CELLS 


..CATHODE 


the  plates  are  joined  by  a  conductor,  current  will  flow  from  the  lead  plate  which 
was  connected  to  the  positive  terminal  of  the  primary  battery  to  the  opposite  lead 
plate,  and  within  the  cell,  in  the  opposite  direction. 

j ,  f ;  When    the    current    flows    from   plate    to 

plate  through  the  electrolyte,  the  plate 
connected  to  the  positive  pole  of  the  battery 
receives  a  brown  coating  of  peroxide  of 
lead  but  the  opposite  plate  becomes  spongy 
or  porous.  Since  the  passage  of  the  cur- 
rent through  the  cell  has  left  one  plate 
unchanged  while  it  has  coated  the  surface 
of  the  other  plate  with  lead  peroxide,  it  is 
reasonable  to  expect  that  if  the  charging 
battery  is  disconnected  and  the  two  dis- 
similar lead  plates  connected  by  a  wire,  a 
current  of  electricity  will  flow  through  the 
external  circuit.  In  fact,  we  now  have  the 
essentials  of  an  ordinary  chemical  cell. 
This  cell  will  continue  to  supply  current 
until  the  lead  peroxide  is  partly  used  up 
and  the  plates  will  gradually  return  to  the 
state  they  were  in  before  the  charging 
process  took  place.  In  order  that  the 
plates  may  be  put  in  condition  to  deliver 


PRIMARY 
CELLS 


Fig.    4 — Simple    Diagram    for 
Cell. 


"Charging"    a    Storage 


current  again,  they  must  be  reconnected  to  the  charging  source  and  a  new  coating  of 
peroxide  of  lead  deposited  upon  the  positive  plate.  It  will  be  seen,  therefore,  that  it  is  not 
really  electricity  which  is  "stored  up"  in  the  storage  cell  but  that  the  current  supplied  to  the 
cell  during  the  charging  process  produces  an  electrochemical  change  which  gives  the  plates 
dissimilar  properties,  and  so  long  as  this  change  is  evident,  there  will  be  a  difference  of 
potential  at  the  terminals  and  therefore  an  electromotive  force.  In  commercial  practice 
storage  cells  are  "charged"  by  electric  dynamos  or  generators  rather  than  by  primary  cells. 

The  electromotive  force  of  primary  cells  varies  from  .06  to  1.5  volts  according 
to  the  nature  of  the  battery  elements 
and  the  grade  of  electrolyte.  The 
electromotive  force  of  the  lead  plate 
secondary  cell  lies  between  2.1  and 
2.6  volts. 

In  electrical  equations,  potential 
or  E.  M.  F.  is  represented  by  the 
letter  E.  Instruments  for  measur- 
ing potential  difference  are  known 
as  "voltmeters. 

16.  Current  Strength  and  Quan- 
tity.— Up  to  this  point  we  have  not 
made  mention  of  the  strength  of 
the  current  or  the  quantity  of  elec- 
tricity flowing  through  a  given  cir- 
cuit. We  have  simply  referred  to  the 
potential  difference  and  the  conse- 
quent electromotive  force  generated 
by  chemical  cells.  Just  as  we  might 
use  in  the  system  of  hydraulics  the 
gallon  per  second  as  a  unit  to  express 
the  quantity  of  water  flowing  from 
a  given  source,  so  in  electrical  cir- 
cuits, we  express  the  quantity  of 

electricity       flowing       by       the       Unit  Fig.   S-Elemental  Storage  Cell. 


8  PRACTICAL  WIRELESS  TELEGRAPHY 

termed  the  coulomb.  We  must  not  confound  the  measure  of  the  total  quantity 
of  electricity  in  a  given  circuit  with  its  strength  or  rate  of  flow.  The  strength  of 
an  electrical  current  should  be  described  as  the  rate  of  flow  of  electricity  through 
a  circuit  per  second  of  time.  When  one  practical  unit  of  quantity  of  electricity 
(one  coulomb)  flows  every  second  continuously,  the  rate  of  flow  or  the  strength 
of  the  current  is  said  to  be  one  ampere;  if  three  unit  quantities  flow  continuously 
every  second,  the  strength  of  the  current  is  three  amperes  and  so  on.  Hence  iJc 
may  define  the  ampere  as  the  quantity  of  electricity  flowing  past  any  point  in  a 
circuit  per  second  of  time. 

The  strength  of  the  current  in  amperes  will  be  seen  to  be  independent  of  the 
length  of  time  the  current  flows  in  a  given  circuit  whether  it  flows  for  a  fraction  of 
a  second,  a  minute,  or  an  hour ;  if  the  quantity  of  electricity  that  would  flow  in  one 
second  is  the  same  in  any  two  or  more  cases  the  current  in  amperes  is  the  same. 

We  may  now  define  the  coulomb  as  the  amount  of  electricity  that  would  pass 
in  one  second  through  a  given  circuit  in  which  the  strength  of  the  current  is  one 
ampere. 

If  a  current  of  one  ampere  flows  every  three  seconds,  the  quantity  of  electricity 
delivered  is  three  coulombs,  or  if  three  amperes  of  current  flow  for  one  second, 
the  quantity  is  also  three  coulombs.  From  this  we  see  that  the  quantity  of 
electricity  in  coulombs  is  equal  to  the  current  strength  in  amperes  multiplied  by 
the  time  it  flows  in  seconds  or, 

0=  I  X  t, 

Where  Q  =  Quantity  of  current  "in  coulombs, 

I  =  Current  in  amperes, 
and  t  =  Time  in  seconds. 

Hence  to  find  out  the  quantity  of  electricity  that  flows  around  a  circuit  in  ten  minutes 
when  the  strength  of  the  current  is  ten  amperes,  we  substitute  the  value  of  I  and  t  in  this 
equation  and  multiply  or,  Q  =  10  X  600  —  6,000  coulombs. 

It  is  more  convenient  in  electrical  practice  to  measure  the  strength  of  the  current  in 
amperes  than  to  compute  the  total  quantity  of  electricity  flowing ;  hence,  when  we  speak  of 
the  current  available  from  a  given  electrical  source,  we  employ  the  unit,  the  ampere,  which 
indicates  the  rate  at  which  it  flows. 

In  electrical  equations  the  ampere  is  represented  by  the  letter  I.  Instruments 
for  measuring  the  strength  of  current  are  called  ampere-meters  or  ammeters. 

17.  Electrical  Resistance. — If  the  terminals  of  a  primary  or  secondary 
cell,  or  a  battery  of  cells,  are  connected  to  a  length  of  copper  wire  and  a  current 
measuring  instrument  such  as  the  ammeter,  connected  in  series  with  the  circuit, 
a  much  greater  reading  or  deflection  of  the  ammeter  will  be  obtained  with  a 
given  length  of  copper  wire  than  with  an  iron  wire  of  the  same  length  and 
diameter.  This  experiment  indicates  that  a  cell  producing  a  constant  E.  M.  F. 
(abbreviation  for  electromotive  force)  can  force  a  very  much  stronger  current 
through  a  copper  wire  than  through  an  iron  wire  of  the  same  proportions.  We 
may  conclude  from  this  that  iron  offers  a  higher  resistance  to  the  passage  of 
electricity  than  copper. 

Resistance  in  electrical  circuits  may  be  defined  as  that  property  of  bodies  which 
opposes  the  flow  of  electric  current.  Just  as  water  passes  with  difficulty  through 
a  small  pipe  of  great  length,  but  very  freely  through  a  large  pipe  of  short  length 
so,  also,  a  small  wire  of  considerable  length  and  poor  conducting  qualities  opposes 
the  flow  of  electricity  considerably,  but  a  good  conductor  of  short  length  and 
large  diameter  offers  but  very  little  resistance. 

All  substances  are  found  to  resist  the  passage  of  electricity  but  the  resistance 
of  metals  is  by  far  the  least.  Of  all  metals,  silver  is  found  to  be  the  best  con- 
ductor, and  it  therefore  possesses  less  resistance  than  copper,  for  example.  In 
fact,  the  ability  of  silver  to  conduct  electricity  is  taken  as  unity  or  the  base  from 
which  the  specific  resistance  of  other  metals  is  computed. 

The  specific  resistance  of  any  material  is  the  resistance  of  a  piece  of  unit  length  and  unit 


THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE.  9 

cross  section  at  an  arbitrarily  adopted  degree  of  temperature.     It  is,  in  fact,  the  resistance 
of  an  inch  cube  of  any  substance  at  the  temperature  of  melting  ice. 

The  following  table  shows  the  relative  resistance  of  chemically  pure  metals  at  the  tem- 
perature of  32  degrees  Fahrenheit.  Resistance  in  Microhms 
Metal                                                                      Relative  Resistance  per  cubic  inch 

Silver  annealed    1.000  .5904 

Copper   annealed    1.063  .6274 

Silver  hard  drawn    1.086  .6415 

Copper  hard  drawn 1.086  .6415 

Gold   annealed    1.369  .8079 

Aluminum    annealed     1.935  1.144 

Zinc  pressed  3.741  2.209 

Platinum   6.022  3.555 

Iron  annealed    6.460  3.814 

Lead   13.05  7.706 

German  Silver 13.92  8.217 

We  learn  from  this  table  that  a  cubic  inch  of  German  silver,  for  instance,  has  a  little 
more  than  13  times  the  specific  resistance  of  a  cubic  inch  of  annealed  silver. 

We  find  by  experiment  that  the  total  resistance  of  a  conductor  varies  directly  as  the 
specific  resistance  and  length,  and  inversely  as  the  cross  sectional  area.  These  quantities 
are  related  in  the  following  way : 

L 

R_  c 
—  «J  > 

A 
where  R  =  the  resistance  in  ohms ; 

L  =  the  length  of  the  conductor ; 
A  =  its  cross   sectional  area; 
S  =  the  specific  resistance  of  the  material. 

Hence  if  we  know  the  length  and  cross  sectional  area  of  a  conductor,  take  the  value  of 
S  from  the  foregoing  table  and  substitute  all  three  values  in  this  formula,  the  total  re- 
sistance is  readily  determined. 

The  resistance  of  metals  is  also  affected  by  temperature;  usually  it  increases  with  in- 
crease of  temperature  but  certain  substances  decrease  their  resistance  under  rise  of  tem- 
perature, an  example  being  carbon  lamp  filaments  and  certain  electrolytic  conductors  such 
as  battery  solutions.  The  hot  resistance  of  the  carbon  filament  of  an  incandescent  lamp 
is  approximately  one-half  the  value  when  cold.  The  resistance  of  a  conductor,  however, 
is  always  constant,  if  the  temperature  remains  constant,  irrespective  of  the  strength  of 
current  flowing  through  it.  If  a  conductor  offers  unit  resistance  to  a  current  of  one 
ampere,  it  offers  the  same  resistance  to  a  current  of  twenty  amperes  provided  the  tempera- 
ture does  not  change  appreciably.  In  most  circuits  encountered  in  practice,  the  rise  of 
temperature  is  not  appreciable,  but  in  case  a  conductor  does  heat  considerably,  the  actual 
resistance  can  only  be  obtained  by  taking  the  temperature  into  account  as  well  as  the 
specific  resistance. 

The  unit  of  resistance  is  called  the  ohm.  The  international  ohm  is  the  re- 
sistance offered  to  the  flow  of  an  unvarying  electric  current  by  a  column  of 
mercury  106.3  centimeters  long,  weighing  14.4521  grams,  at  a  temperature  of  32 
degrees  Fahrenheit. 

Examples  of  conductors  in  ordinary  electrical  practice  having  approximately  this  value 
of  resistance  follow : 

1  ohm  =  250  ft.  of  No.  16  B.  and  S.  copper  wire,  which  is  l-20th  of  an  inch  in  diameter. 

1  ohm  =r  1,000  ft.  of  No.  10  B.  and  S.  copper  wire,  which  is  l-10th  of  an  inch  in  diameter. 

One  thousand  feet  of  No.  32  B.  and  S.  bare  copper  wire  has  resistance  of  170.7  ohms. 
In  electrical  equations  resistance  is  expressed  by  the  letter  R. 

18.  Grouping  of  Electrical  Cells. — Battery  cells  may  he  grouped  in  three 
ways : 

(1)  In  series; 

(2)  In  parallel; 

(3)  In  series  multiple  or  series  parallel. 

Keeping  in  mind  the  units  for  expressing  the  strength  and  pressure  of  an 
electric  current,  we  shall  now  see  how  the  grouping  of  cells  in  various  ways  affects 
the  current  and  pressure  available  for  a  given  external  circuit. 


10 


PRACTICAL   WIRELESS   TELEGRAPHY 


rL5Vi 


A  series  connection  is  made  by  joining  the  positive  pole  (the  carbon  plate)  of 
one  cell  to  the  negative  pole  (zinc  plate)  of  the  next  cell,  as  shown  in  the  diagram, 

Fig.  6.  The  upper  part  of  this  dia- 
gram shows  two  chemical  cells  con- 
nected in  series  and  the  lower  figure 
shows  ten  cells  connected  in  series 
as  they  would  be  represented  in  or- 
dinary electrical  diagrams. 

In  the  upper  diagram  of  Fig.  6,  current 
flows  from  the  carbon  plate  of  the  cell 
through  the  switch  S,  through  a  concen- 
trated resistance  coil  R  (which  may  be  of 
German  silver  or  other  metal  of  high 
specific  resistance)  to  the  zinc  plate  of  the 
right  hand  cell.  The  circuit  continues 
from  the  zinc  plate  through  the  electrolyte 
to  the  carbon  plate,  from  the  carbon  plate 
to  the  zinc  plate  of  the  left  hand  cell,  and 
finally  through  the  electrolyte  to  the  carbon 
plate  from  which  the  flow  originally  began. 
When  the  switch  S  in  this  diagram  is 
opened,  the  circuit  from  the  battery  cells 
is  said  to  be  broken  or  interrupted;  but 


S 

U- 

1.5V 

vvvvvvv 

R 

1                r 

c 

t       ii 

§& 

I 

1 

c 

1 

1 

1 

1 

1 

i«; 

I 

i 

I 

I 

I 

i! 

* 

r^ 

Fig.    6 — Electric  Cells  Joined  in   Series. 


when  switch  S  is  closed,  the  circuit  it  called 


a  closed  circuit,  current  flowing  freely  from  the  positive  to  the  negative  pole  of  the  battery. 
Now  the  total  resistance  of  this  circuit  is  made  up  of  the  resistance  coil  R,  the  re- 
sistance of  the  connecting  leads  and  the  internal  resistance  of  the  battery  cells,  that  is  the 
resistance  of  the  electrolyte  from  plate  to  plate.  The  strength  of  the  current  flowing 
through  this  circuit,  as  will  be  explained  in  detail  further  on,  is  governed  by  the  total 
E.  M.  F.  of  the  cells  and  the  total  resistance  of  the  circuit  and  since  the  internal  resistance 
of  ordinary  primary  cells  is  rather  high,  it  cannot  be  ignored  in  the  grouping  of  cells. 

When  cells  are  connected  in  series,  the  total  electromotive  force  is  that  of  one  cell 
multiplied  by  the  number  of  cells  in  the  group  (provided  all  cells  have  identical  potential)  ; 
but  the  strength  of  the  current  will  not  exceed  that  of  a  single  cell,  and  more  likely  will 
be  less,  due  to  the  fact  that  the  total  resistance  of  the  circuit  increases  as  more  cells  are 
added  (o  the  battery.  Grouping  of  the  cells  in  either  series  or  parallel  affects  the  total 
internal  resistance  as  follows :  When  a  number  of  cells  in  a  battery  are  connected  in  series, 
the  total  internal  resistance  is  equal  to  the  sum  of  the  internal  resistances  of  all  the  cells. 
When  a  number  of  like  cells  are  connected 
in  parallel,  the  total  internal  resistance  is 

equal  to  the  resistance  of  one  cell  divided  1-5  V 

by  the  number  of  cells  in  the  battery. 

A  parallel  connection  is  made  by 
connecting  the  positive  terminal  of  one 
cell  with  the  positive  terminal  of  an- 
other cell  and  the  negative  terminal 
of  the  first  cell  with  the  negative 
terminal  of  the  second  cell  as  in  the 
diagram  Fig.  7.  The  left  hand  figure 
shows  the  direction  of  the  flow  of  cur- 
rent of  two  cells  connected  in  parallel 
and  by  this  connection,  the  total 
electromotive  force  of  the  cells  is  no 
more  than  that  of  a  single  cell  but  the 
current  available  (in  amperes)  is  that 
of  one  cell  multiplied  by  the  number 
of  cells  in  the  group. 

Applying  this  to  the  right  hand  dia- 
gram of  Fig.  7  where  four  cells  are  con- 


v VWWWV — — J 


+ 

•  — 

1. 

5V 

1 

5V 

fc 

5V 

+ 

I~ 

R 

v VWWWV— 

9  OHMS 


Fig.  7 — Electric  Cells  Joined  in  Parallel. 


THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE. 


11 


UNIT  A  3- 1.5V -4.5V 


nected  in  parallel,  the  potential  difference  across  the  terminals  is  1.5  volts  and  the  current 
available  in  amperes  will  be  4  X  15  or  60  amperes.  If  the  resistance  of  R  is  rather  high, 
of  the  order  of  a  few  hundred  ohms,  this  potential  difference  can  be  measured  by  connect- 
ing a  voltmeter  across  the  terminals.  It  should  indicate  E.  M.  F.  of  \y2  volts.  We  see 
from  this  that  the  final  effect  of  a  parallel  connection  in  the  case  cited  is  the  same  as  if  4 

zinc  plates  joined  together  and  4  carbon 
plates  joined  together  were  immersed  in  a 
single  battery  tank. 

A  series  parallel  connection  of 
electrical  cells  is  shown  in  the  dia- 
gram, Fig-.  8,  where  groups  A  and  B 
each  *  consist  of  three  battery  cells 
joined  in  series  and  shunted  by  the 
resistance  R. 

Since  groups  A  and  B  consist  of  three 
cells  connected  in  series,  then  (in  accord- 
ance with  the  statements  made  concerning 


III 


UNIT  "B  3»I.5V«4.5V 


III 


R  =  45     OHVAS 

-vvwww 

4.5  VOLTS 


Fig.   S; — Cells   Grouped   in    Series   Parallel. 


series  connection  of  cells  in  previous  para- 
graphs) the  voltage  of  each  group  will  be 
3  X  1.5  or  4.5  volts,  but  the  current  avail- 
able from  either  groups  A  or  B  will  not 
be  greater  than  that  of  a  single  cell  in 
either  unit.  Assuming  that  each  group  is 


capable  of  delivering  current  of  15  amperes,  there  will  be  available  for  the  external  circuit 
15  -f-  15  or  30  amperes,  but  the  total  electromotive  force  of  the  two  groups  is  that  only  of 
one  group  or  4.5  volts. 

When  the  resistance  of  the  external  circuit  is  small  in  comparison  with  the 
internal  resistance  of  the  cells,  there  is  no  advantage  in  the  series  connection  be- 
cause the  flow  of  current  will  be  governed  by  the  resistance  of  the  battery  rather 
than  by  the  external  circuit.  In  this  case  parallel  grouping  of  the  cells  is  most 
desirable.  When  the  external  resistance  is  large  in  comparison  with  the  internal 
resistance  of  a  single  cell,  the  cells  are  most  advantageously  connected  in  series. 
It  is  thus  plain  that  the  connection  to  be  adopted  will  depend  upon  the  resistance 
of  the  external  circuit  as  compared  to  that  of  the  internal  resistance  of  the  battery. 
In  the  majority  of  cases,  the  most  efficient  grouping  is  the  one  where  the  internal 
resistance  of  the  cells  about  equals  the  resistance  of  the  external  circuit. 

19.  Ohm's  Law  and  Practical  Application. — The  relation  between  electro- 
motive force,  current  strength  and  the  resistance  of  an  electrical  circuit  is  dis- 
closed by  Ohm's  law  which  states  that  the  strength  of  the  current  in  amperes  in 
any  given  circuit  is  directly  proportional  to  the  E.  M.  F.  and  inversely  proportional 
to  the  resistance,  or  using  the  symbols  of  the  previous  paragraph, 

E 

I  =  - 
R 

which  may  be  written 

Volts 

Amperes  =  

Ohms 

The  student  cannot  overestimate  the  importance  of  this  law  because  only  as  it 
is  thoroughly  understood  can  electrical  circuits  be  handled  and  cared  for  intelli- 
gently. Applying  the  law  practically,  if  an  E.  M.  F.  of  6  volts  is  applied  to  a  circuit 
having  a  total  resistance  of  3  ohms,  the  current  strength  in  amperes  is  obtained  as 
follows : 

6 

I  =  -    =z  2  amperes 
3 

By  transposing  this  equation  we  may  write 


12  PRACTICAL   WIRELESS   TELEGRAPHY 

E  =  I  X  R 
E 

or  R  =  — 

I 

It  is  plainly  evident  that  if  we  know  any  two  of  the  quantities  involved  in  this 
expression,  the  third  may  be  readily  determined. 

To  illustrate :  If  the  flow  of  current  in  a  given  circuit  is  2  amperes  and  its  total  re- 
sistance 220  ohms,  the  E.  M.  F.  applied  to  set  up  this  value  of  current  must  have  been. 
220  X  2  =  440  volts  (E  =  I  X  R). 

As  a  second  illustration  we  may  take  the  case  of  an  ordinary  carbon  filament  carbon 
lamp  which  takes  0.5  amperes  under  pressure  of  110  volts.  According  to  this  law,  the 

110  E 

resistance  of  the  filament  must  be  or  220  ohms   (R  =  — ). 

0.5  I 

We  learn  from  Ohm's  equation  that  to  increase  the  flow  of  current  through  a 
circuit  of  fixed  resistance  we  must  increase  the  voltage.  If  the  voltage  be  doubled, 
the  flow  of  current  is  doubled  and  so  on.  By  the  same  law  if  the  flow  of  current 
through  a  given  device  and  the  pressure  across  its  terminals  can  be  measured,  the 
resistance  in  ohms  is  obtained  by  simply  dividing  the  pressure  in  volts  by  the 
current  in  amperes.  • 

Ohm's  law  applied  to  the  circuit  of  Fig.  6  yields  the  following  results :  If  the  coil  R 
has  a  resistance  of  9  ohms  and  the  E.  M.  F.  of  the  cells  is  3  volts,  the  strength  of  the  cur- 

3 
rent  through  R  =  —  =  0.33  amperes   (assuming  the  internal  resistance  of  the  cells  and 

9 

connecting  wires  to  be  negligible).     If  R  had  18  ohms  resistance,  0.166  amperes  would  flow 
through  the  circuit.     If,  in  the  left  hand  drawing,  Fig.  7,  R  has  9  ohms  resistance  and  the 

1.5 

E.  M.  F.  is  V/2  volts,  the  current  =  —  =  0.16  amperes.    Also  if  R  in  Fig.  8  had  resistance 

9 

4.5 
of  9  ohms,  the  flow  of  current  would  be  —  or  0.5  amperes. 

9 

If  a  number  of  electrical  devices  are  connected  in  series  as  in  diagram,  Fig.  9,  the  cur- 
rent through  each  element  is  the  same,  irrespective  of  its  resistance.  In  this  diagram  a 
source  sf  direct  current  potential  B,  of  100  volts  is  applied  to  the  circuit  comprising  an 
electric  lamp  L,  of  180  ohms,  a  resistance  coil  R,  of  50  ohms,  and  a  telegraph  sounder  S,  of 
4  ohms.  We  may  calculate  the  current  flowing  at  any  point  through  the  circuit  such  as  at 
A,  by  first  determining  the  total  resistance.  This,  exclusive  of  the  cells  and  connecting 

100 
wires  leading  therefrom,  is  180  +  50  +  10  =  240  ohms;  the  current  in  amperes  —  - 

240 
0.41  amperes. 

It  is  to  be  especially  noted  that  the  strength  of  the  current  through  all  the  elements  of 
this  circuit  is  the  same,  irrespective  of  the  resistance  of  the  individual  elements  but  the 
current  is  governed  principally  by  the  greater  resistance,  that  of  the  lamp  L. 

If  a  voltmeter  be  connected  to  the  terminals  of  any  of  the  various  resistance 

elements  of  the  circuit  (see  V  in  Fig. 
^-;fv  9),. a  difference  of  potential  or  elec- 

(TD          r~VV~~~!  tromotive  force  will  be  found  to  exist 

across  the  terminals  that  varies  as  the 
resistance  and  the  strength  of  the  cur- 
rent.    The   electromotive    force   may 
Ttenpl    be  calculated  directly  by  Ohm's  law 
si    -^-  anil    if  the  resistance  and  the  current  are 


R 
50  OHMi 


I     —  j~—      known         jf     the     current     flowing 

L  10  OHMS               ,                                               ,                                                             .                                 . 

^^  through    each    resistance    element    is 

— -""^^  0.41  amperes,  the  pressure  in  volts  is 


Fig.  9— Electrical  Devices  Connected  in  Series.       obtained   by   multiplying  the   current 


THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE. 


13 


strength  in  amperes  by  the  resistance  in  ohms.  Thus  the  potential  difference  across 
R  =  0.41  X  50  =  20.5  volts;  similarly  across  L  =-  180  X  0.41  -  73.8  volts  (the 
calculation  being  made  oruthe  assumption  that  the  internal  resistance  of  the 
cells  is  zero). 

20.  Divided  Circuits. — A  divided  or  shunt  circuit  is  an  additional  circuit 
provided  at  any  part  of  a  circuit  through  which  the  flow  of  current  sub-divides. 
One  branch  of  such  a  circuit  is  said  to  be  in  multiple  or  in  parallel  with  the  other 
branch  or  branches. 

Fig.  10  represents  a  divided  circuit  of  3  branches,  R-l,  R-2  and  R-3.  If  resistances 
R-l,  R-2  and  R-3  are  equal,  the  current  flowing  from  A  to  B  will  divide  equally  between 

the  3  branches.     If  a  current  of  9  amperes 

+  is  flowing  in  the  main  circuit  as  indicated 

"  by  the  ammeter  A,  3  amperes  will  flow 
through  each  branch.  If  the  resistances 
are  unequal,  the  current  divides  inversely 
as  their  relative  resistance. 

The  current  in  the  branches  of  the 
divided  circuit,  Fig.  10,  can  be  determined 
by  finding  the  voltage  across  the  terminals 
of  each  branch,  and  dividing  the  result  by 
the  resistance  of  each  branch. 

Thus    the    current    in    branch    R-l    = 
E  E  E 

Fig.    10 — Diagram    Showing    Branch    Electrical    Circuits.  —  — ?     R-2     — and     in     R-3     '=.    -   — . 

R-l  R-2  R-3 

100  100  100 

R-l  passes  -    -  or  3.33  amperes;  R-2  passes  -    -  or  5  amperes  and  R-3,  -    -  or  10 

30  20  10 

amperes.  An  ammeter  connected  in  series  with  the  circuit  as  at  point  C  should  indicate 
3.33  -f  5  -f-  10  or  18.33  amperes.  (Resistance  of  the  connecting  leads  being  ignored). 

When  several  resistances  are  connected  in  parallel  their  joint  resistance  is  computed  as 
follows : 


R-l 
30  OHMS 


R-2 

20  OHMS 


R-3 

'0  OHMS 


R  = 


1 


where  R  =  the  joint  resistance. 
the  3  elements  is  equal  to: 


R-l         R-2       R-3 
Hence,  in  the  circuit  of  Fig.   10,  the  joint  resistance  of 


1 

=  —  =r  5.4  ohms. 
11 


30       20        10        60 

It  is  now  clear  that  two  or  -more  resistances  in  parallel  will  conduct  an  electric 
current  more  freely  than  one,  and  the  joint  resistance  of  several  resistances  in 
parallel  is  less  than  the  resistance  of  the  smaller  one. 

When  a  nnmber  of  resistances  are  connected  in  series  their  joint  resistance  is 
the  sum  of  several  resistances  taken  separately. 

21.  Electrical  Work. — When  a  current  of  electricity  flows  through  a  con- 
ductor, it  encounters  frictional  resistance  and  a  certain  amount  of  the  energy  is 
transformed  into  heat.  The  heat  of  a  conductor  under  certain  conditions  may  be 
so  great  that  unless  due  precaution  is  taken,  the  wire  will  melt.  We  find  that 
when  an  electric  current  has  passed  through  a  substance,  the  development  of  heat 
is  proportional, 

(1)  To  the  time  during  which  the  current  flows; 

(2)  To  the  square  of  the  current; 

(3)  To  the  resistance  of  the  conductor. 


14  PRACTICAL  WIRELESS  TELEGRAPHY 

This  may  be  expressed : 

J  ==  I2  X  R  X  T, 
where  J  —  the  electrical  energy  expended  in  the  form  of  heat  in  joules. 

The  joule  is  defined  as  that  amount  of  energy  which  is  expended  during  one 
second,  by  current  of  one  ampere  flowing  through  a  resistance  of  1  ohm  and  the 
joule  per  second  is  the  practical  unit  of  electrical  power  which  has  been  named 
the  ivatt. 

joules 
Since  power  is  the  rate  of  doing  work  per  unit  of  time,  watts  =  -        — . 

time 

Hence  if  2,000  joules  of  electrical  work  are  done  in  twenty  seconds,  the  power  exerted 
2000 
is  -    -  =  100  joules  per  second  —  100  watts.     Power  may  also  be  expressed  in  the  units 

20 

of  pressure  and  current  strength.  The  power  in  watts  in  a  given  circuit  in  which  direct 
current  is  flowing  is  equal  to  the  product  obtained  by  multiplying  the  current  in  amperes 
hy  the  electromotive  force  in  volts  or 

W  =  I  X  E. 

Hence,  if  in  any  given  direct  current  circuit  we  measure  the  pressure  by  a 
voltmeter,  and  the  current  strength  by  an  ammeter,  the  power  in  watts  is  obtained 
by  multiplying  together  the  readings  of  each  instrument. 

22.  Electrical  Horsepower. — The  unit  of  mechanical  work  is  a  foot 
pound.  It  is  the  work  done  in  raising  a  mass  of  1  Ib.  against  the  force  of  gravity 
through  a  distance  of  1  ft.  Work  done  at  a  rate  equal  to  33,000  ft.  Ibs.  per  minute 
is  called  the  horsepower  (abbreviated  H.  P.). 

One  Mechanical  H.  P.  =  33,000  ft.  Ibs.  per  minute  =  550  ft.  Ibs.  per  second.  Also  it 
can  be  shown  that  1  joule  =  0.7373  ft.  Ibs.,  hence,  1  joule  per  second  or  1  watt  =  0.7373 

1  watt 

Ibs.  per  second.     Therefore  1  ft.  Ib.  per  second  =  -      — . 

0.7373 

1  H.  P.        1  watt  550 

Since  1  ft.  Ib.  per  second  =  —       —  or  —      — ,  therefore  1  mechanical  H.  P.  = 


550  0.7373  0.7373 

746  watts. 

Now  746  watts  =  1  mechanical  H.  P.,  therefore 

W 

H.  P.  =  or, 

746 

IXE 

H.  P.  = 

746 

Where  I  =  the  current  in  amperes,  E  —  pressure  in  volts. 
For  example,  an  electric  motor  requires  30  amperes  current  at  pressure  of  110  volts;  its 

110X30         3300 

rating  in  H.  P.  =  -  -  =  4.4  horsepower. 

746  746 

1  kilowatt  =  1000  watts  =  1.34  H.  P. 
1  H.  P.  =  746  watts  =  .746  K.  W. 

23.  Definition  of  Electrical  Units. — The  practical  units  of  electricity  may 
be  defined  as  follows: 

The  practical  unit  of  electromotive  force  is  the  volt,  and  by  definition  the  volt 
is  that  E.  M.  F.  required  to  maintain  the  flow  of  current  of  one  ampere  through  a 
resistance  of  one  ohm. 

The  practical  unit  of  current  strength  is  the  ampere,  and  it  is  that  strength  of 
current  maintained  by  an  E.  M.  F.  of  one  volt  through  a  resistance  of  one  ohm. 

The  ohm  is  the  unit  of  resistance  and  is  such  resistance  of  conductor  or  circuit 
that  permits  the  passage  of  a  current  of  one  ampere  under  an  E.  M.  F.  of  one  volt. 


THE  PRODUCTION  OF  ELECTROMOTIVE  FORCE. 


15 


The  unit  of  current  quantity  is  the  coulomb  which  is  the  quantity  of  electricity 
flowing  in  a  circuit  when  one  ampere  passes  a  given  point  during  one  second  of 
time. 

The  watt  is  the  unit  of  electrical  power  and  is  equal  to  one  joule  per  second. 
It  is  the  power  of  a  current  of  one  ampere  flowing  under  electric  pressure  of 
one  volt. 

In  connection  with  these  units,  the  prefixes  of  kilo,  micro  and  milli  are  employed,  mean- 

1  1 

ing  respectively,   1,000  times,  -  -  of  and  -    -  of.     Thus  a  kilo-volt  =  1,000  volts;  a 

1,000,000  1,000 

1  1 

micro-ampere  —  -  -  ampere;  and  a  milli-volt  =  -    -  of  a  volt. 

1,000,000  1,000 

24.  Current  Output  and  Voltage  of  Various  Devices. — For  students' 
information,  we  may  review  here  the  values  of  voltage  and  current  to  be  expected  from 
various  current  sources  and  circuits  in  daily  use.  For  example,  primary  cells  of  various  types 
generate  an  E.  M.  F.  varying  between  0.6  to  1.75  volts.  The  current  output  varies  with  the 
size  and  nature  of  the  elements,  lying  between  5  and  30  amperes  for  common  sizes.  Storage 
cells  generate  an  E.  M.  F.  between  2.08  and  2.6  volts.  The  rated  current  output  may 
vary  from  5  to  200  amperes,  depending  upon  the  size  of  the  cell. 

Generators  or  dynamos  are  constructed  to  supply  potentials  from  4  to  6,000  volts,  the 
latter  value  being  rarely  exceeded.  Certain  types  of  generators,  for  instance  those  used  in 
electroplating  establishments,  may  have  a  current  output  of  10,000  amperes,  with  an  E.  M.  F. 
of  4  to  8  volts.  The  electric  lighting  wires  of  homes  generally  carry  current  at  pressure  of 
110  volts,  either  direct  or  alternating  current.  Transmission  lines  for  carrying  large 
amounts  of  power  over  great  distances  may  have  voltages  as  high  as  200,000  volts,  but  the 
strength  of  the  current  is  comparatively  small.  The  potential  of  trolley  wires  is  generally 
about  550  volts.  Voltages  in  excess  of  110  volts  are  considered  dangerous  to  human  life, 
particularly  those  in  excess  of  500  volts. 


Fig.    lOa — Portable    Wireless    Transmitting   and    Receiving    Set   for   Junior    Military   Organizations. 


Note : — The  physical  standard  for  the  ohm  has  been  noted.  The  standard  for  the 
strength  of  current  is  an  arbitrary  one.  It  is  found  that  if  a  silver  and  a  platinum  electrode 
are  dipped  in  a  neutral  solution  of  silver  nitrate  (consisting  of  15  parts  by  weight  of  silver 
nitrate  and  18  parts  of  water)  a  steady  current  of  one  ampere  flowing  from  the  silver  to 
the  platinum  will  deposit  .001118  grams  of  silver  on  the  platinum  per  second. 

The  standard  for  the  volt  is  the  Weston  Cadmium  cell  which  has  an  electromotive  force 
of  1.018  volts  at  a  temperature  of  20  degrees  Centigrade. 


PART  III. 
ELECTROMAGNETIC  INDUCTION. 

THE  DYNAMO— THE  FLOW  OF  ALTERNATING  CURRENT. 

25.  ELECTROMAGNETISM.  26.  MAGNETIC  FIELD  ABOUT  Two 
PARALLEL  CONDUCTORS.  27.  THE  SOLENOID.  28.  INDUCED  CUR- 
RENTS. 29.  MUTUAL  INDUCTION.  30.  SELF-INDUCTION. 
31.  VALUE  OF  INDUCED  E.  M.  F.  32.  THE  DYNAMO.  33.  DE- 
TERMINATION OP  FREQUENCY.  34.  STRENGTH  OF  MAGNETIC 
FIELD.  35.  DIAGRAM  OF  AN  ALTERNATING  CURRENT  DYNAMO. 
36.  DIRECT  CURRENT  DYNAMO.  37.  SHUNT,  SERIES  AND  COM- 
POUND WOUND  DYNAMOS.  38.  THE  ELECTRIC  MOTOR.  39.  THE 
EFFECT  OF  COUNTER  ELECTROMOTOR  FORCE.  40.  MOTOR  WITH 
DIFFERENTIAL  FIELD  WINDING.  41.  DYNAMO  AND  MOTOR  ARMA- 
TURES. 42.  DEVELOPMENT  OF  ARMATURE  WINDINGS.  43.  THE 
ALTERNATING  CURRENT  TRANSFORMER.  44.  ELECTROSTATIC  CA- 
PACITY. 45.  REACTANCE  AND  IMPEDANCE.  46.  CAPACITY  RE- 
ACTANCE. 47.  LAG  AND  LEAD  OF  ALTERNATING  CURRENT.  48.  EF- 
FECTIVE VALUE  OF  ALTERNATING  E.  M.  F.  AND  CURRENT. 
49.  MEASURING  INSTRUMENTS  OR  ELECTRIC  METERS.  50.  INDUC- 
TION COIL.  51.  PRACTICAL  ELECTRIC  CIRCUITS. 

25.  Electromagnetism.     An  explanation  of  some  of  the  more  important 
phenomena  surrounding  a  current  carrying  conductor  follows :     If  a  conductor 

through  which  a  current  of  electricity 
_  is  passing  is  laid  parallel  to  and  above 
1  a  compass  needle  as  in  Fig.  11,  the 
needle  will  tend  to  turn  at  a  right 
angle  to  the  conductor,  but  if  the  cur- 
rent is  turned  off,  the  needle  will  re- 
turn to  its  original  position.  As  we 
have  previously  mentioned  a  magnet 
suspended  freely  will  tend  to  lie 
parallel  to  a  given  magnetic  field, 
hence,  it  follows  from  this  experi- 

Fig.     11 — Deflection     of     Compass    Needle     by     Electric  ,        n  r  ,1 

Current.  meiit  that  the  flow  of  current  through 

the  wire  of  (Fig.  11)  must  have  set 

up  a  magnetic  field  and  the  direction  of  which  is  evidently  at  right  angles  to  the 
conductor.  j 

//  the  current  in  a  horizontal  conductor  is  flowing  towards  the  north,  and  a 
compass  is  placed  under  the  wire,  the  north  pole  of  the  needle  will  be  deflected 
towards  the  west;  if  the  compass  is  placed  over  the  zvire,  the  north  pole  of  the 
needle  will  be  deflected  towards  the  east.  Or,  if  the  current  is  reversed  in  the 
conductor,  the  needle  will  point  in  the  opposite  direction  in  each  case  respectively. 
From  this  and  other  experiments  we  deduce  that  if  the  current  in  a  conductor  is 
flowing  away  from  the  reader,  as  in  Fig.  12a,  the  direction  of  the  lines  of  force 
will  be  around  the  conductor  in  the  direction  of  the  hands  of  a  clock.  If,  on  the 
other  hand,  the  current  flows  towards  the  reader  as  in  Fig.  12b,  the  direction  of 
the  lines  of  force  will  be  around  the  conductor  in  the  direction  opposite  to  the 
movement  of  the  hands  of  a  clock  or  counter  clockwise. 


ELECTROMAGNETIC   INDUCTION. 


17 


26.  Magnetic  Field  About  Two  Parallel  Conductors. — The  magnetic  fields 
of  two  parallel  conductors  are  either  mutually  attractive  or  repellent,  according 

to   the   direction   of   the   current   in 
each. 

In  the  diagram,  Fig.  13,  the  current 
in  the  left  hand  wire  is  flowing  away 
from  the  reader,  but  in  the  right  hand 
wire  towards  the  reader.  Since  the  gen- 
eral direction  of  the  lines  of  force  is  op- 
posite in  either  wire,  their  magnetic 
fields  are  in  opposition  or  in  repulsion. 
In  the  diagram,  Fig.  14,  current  is  as- 
sumed to  be  flowing  in  both  wires  in  the 
same  direction  and  since  the  lines  of  force 
have  the  same  general  direction,  they 
combine  and  coalesce  as  shown  by  the 
outer  lines. 

27.  The  Solenoid. — If  a  number 
of  turns  of  wire  be  wound  in  a 
spiral,  as  in  Fig.  15,  the  lines  of 
force  generated  by  each  turn  of 
wire  will  unite  with  those  set  up  by 
adjacent  turns.  The  lines  of  force 
inside  each  turn  will  have  the  same 
general  direction,  forming  several 
long  lines  of  force  that  may  be  said 
to  pass  through  the  entire  helix. 
These  lines  pass  out  of  the  coil  at  one 
end  and  enter  at  the  other  end,  just 
as  in  the  case  of  the  bar  magnet 
described  in  Part  1. 

If  the  general  direction  of  the 
lines  of  force  inside  this  coil  is  from 
right  to  left,  the  left  hand  end  will 
be  a  north  pole,  the  opposite  end,  a 
south  pole.  The  polarity  of  the  coil 
may  always  be  determined  if  the  di- 
rection of  the  current  is  known. 
The  rule  is  that  if  in  looking  at  the 
end  of  the  coil,  the  current  flows  around  its  turns  clockwise,  the  end  nearest  to  the 
observer  will  be  a  south  pole,  but  if  the  current  flows  in  the  opposite  direction,  it 
will  be  a  north  pole. 

A  helix  consisting  of  a  number  of  turns  through  which  current  flows  is  known 
as  a  solenoid.  We  see  from  the  foregoing  that  a  solenoid  has  north  and  south 
poles  and,  in  fact,  possesses  all  the  properties  of  a  permanent  steel  magnet  with 
the  advantage  that  the  magnetism  in  the  case  of  the  solenoid  is  entirely  under 
control. 

The  strength  of  the  magnetic  field  of  a  solenoid  is  proportional  to  the  strength 
of  the  electric  current  passing  through  it  and  the  number  of  turns  of  wire  com- 
posing the  coil,  but  the  magnetizing  power  may  be  increased  from  200  to  2,000 
times  by  merely  inserting  an  iron  core  or  bar  of  soft  iron  within  it. 

In  order  that  the  phenomena  of  electromagnetic  induction  to  be  explained  later 
may  be  better  understood,  the  expansion  and  contraction  of  the  magnetic  field 
around  a  current  carrying  coil  should  be  considered.  //  direct  current  of  unvary- 
ing strength  Hows  through  the  solenoid,  the  lines  of  force  stand  still  when  the 
now  of  current  is  fully  established.  If  the  rate  of  How  of  current  is  increased  or 
decreased  the  lines  of  force  increase  or  decrease  accordingly,  -or  stated  in  another 


Fig.  12a — Showing  Lines  of  Force  Around  a  Conductor 
with  the  Current  Flowing  Away  from  Reader. 


18 


PRACTICAL  WIRELESS  TELEGRAPHY. 


way,  when  the  current  rises,  the  lines  of  force  move  away  from  the  wire  but 
when  the  current  falls,  the  lines  of  force  collapse  back  upon  the  wire. 

The  general  direction  of  the  magnetic 
field  around  a  horse  shoe  magnet  is  shown 
in  Fig.  16.  If  the  direction  of  the  flow  of 
current  from  the  battery  around  the  convolu- 
tions of  the  two  coils  is  as  indicated,  the 
left  hand  pole  has  north  magnetism  and  the 
right  hand  pole  has  south  magnetism.  A 
piece  of  soft  iron  A  placed  near  to  the  tips 
of  the  poles  will  be  forcibly  drawn  to  them 
and  will  only  be  released  when  the  current 
is  turned  off.  If  a  coil  of  resistance  wire 
R,  such  as  a  German  silver  resistance  regu- 
lator, is  connected  in  series  with  the  wind- 
ings, the  strength  of  the  magnetic  field  can 
be  closely  regulated.  If  high  values  of  re- 
sistance are  inserted,  the  current  may  be  re- 
duced to  a  degree  that  the  magnet  will  barely 
attract  the  piece  A.  Variation  of  the  cur- 
rent flow  would  affect  the  field  of  a  straight 
solenoid  winding  in  the  same  manner.  The 
point  to  be  taken  from  this  is  that  whenever 
electromagnets  are  employed  for  mechanical 
work  such  as  lifting  masses  of  iron  or  for 
exciting  the  magnets  of  a  dynamo  or  motor, 
the  strength  of  the  field  can  be  regulated 
over  certain  limits  by  a  simple  variable  re- 
sistance. 

If  a  horse  shoe  of  hard  tempered  steel  be 
inserted  in  the  magnetic  windings  in  place 
of  the  soft  iron  core  and  allowed  to  remain 
for  a  few  seconds,  upon  removal  it  will  be 
found  to  be  permanently  magnetized. 

We  have  explained  that  the  direction  of 
the  magnetic  field  around  a  conductor  de- 
pends upon  the  direction  of  the  flow  of  cur- 
rent. It  is  clear  that  if  the  current  of  the 
magnet  in  Fig.  16  is  reversed  the  polarity 
will  be  reversed  as  shown  in  Fig.  17. 

The  strength  of  the  magnetic  field 
about  a  solenoid  can  be  varied  by 
fluxes  of  opposite  directions  as  shown  in  Fig.  18.  The  solenoid  windings 
A  and  B  are  wound  in  opposite  directions  connected  in  series  and  finally 
to  the  terminals  of  the  battery.  Since  current  flows  through  the  two  coils  in 
opposite  directions  their  magnetic  fields  are  repellent  and  if  the  coils  are 
telescoped  together  (one  within  the  other)  the  magnetic  field  will  be  nearly 
destroyed.  If  the  two  coils  are  partially  telescoped,  the  resultant  magnetic  field 
varies  accordingly.  Advantage  is 
taken  of  this  principle  in  construct- 
ing an  instrument  known  as  the 
variometer,  which  is  particularly 
useful  for  tuning  wireless  telegraph 
circuits. 

The  electromagnet  in  some  form  is 
employed  in  nearly  all  electrical  ma- 
chinery, and,  therefore,  the  laws  r 

J .'  r    i  «  til     Fig.   13— Lines  of  Force  About  Two  Conductors  Carrying 

governing      magnetic      fields      Should  Current  in    Opposite    Directions, 


Fig.    12h — Showing    Lines    of    Force    Around    a    Con 
ductor   with   Current   Flowing  Towards  Reader. 


ELECTROMAGNETIC   INDUCTION. 


19 


have  careful  attention.     Study  of  the  phenomenon  of  magnetic  induction  is  par- 
ticularly important  as  it  is  encountered  at  many  points  in  a  wireless  telegraph  set. 

28.  Induced   Currents. — We  have   already  seen   that  a  magnetic   field  in- 

variably accompanies  the  flow  of  electricity 
through  a  conductor  and  conversely  we  find 
that  whenever  a  conductor  is  moved 
through  a  magnetic  field,  an  electromotive 
force  will  be  induced  therein,  and  a  flow  of 
current  will  take  place  if  the  conductor 
forms  a  closed  or  continuous  circuit.  This 
is  the  fundamental  principle  upon  which 
the  operation  of  the  dynamo  or  generator 
is  based. 

Experiment  reveals  that  the  production 
Fig.     14 — Lines    of    Force    Around    Two     Conductors   of  the   E.   M.  F.  is  conditioned   by  the  fol- 

lowing  rule:     The  motion  of  the  coil  must 

take  place  in  such  a  way  as  to  change  the  total  number  of  magnetic  lines  of  force  which 
are  enclosed  by  the  coil. 

Eor  instance,  simply  moving  a  coil  in  a  uniform  field  from  one  position  to  another,  so 
that  the  lines  of  force  enclosed  by  the  coil  remain  of  constant  number,  will  not  induce  a 
flow  of  current,  but  if  the  coil  is  rotated,  for  example,  so  that  the  lines  of  force  enclosed  by 
it  either  increase  or  diminish,  an  E.  M.  E.  will  be  induced  which  varies  according  to  the 
rate  at  which  the  lines  of  force  change. 

The  induction  of  current  by  a  magnetic  field  threading  in  and  out  of  a  coil  can  be 
shown  by  a  simple  experiment.  If  the  terminals  of  a  solenoid  wound  with  fine  wire  are 
connected  to  a  current  indicating  device,  such  as  a  galvanometer,  and  a  permanent  bar 
magnet  plunged  into  the  interior  of  the  winding,  a  momentary  deflection  of  the  galvanometer 
is  observed.  If  the  bar  remains  within  the  coil  there  is  no  further  movement  of  the  current 
indicator.  If  the  bar  be  suddenly  withdrawn,  the  galvanometer  gives  a  second  deflection  in 
the  direction  opposite  to  that  cited  in  the  first  instance.  This  experiment  proves  that  the 
cutting  of  the  flux  through  a  coil  of  wire  induces  a  current  therein  and  that  the  direction 
of  the  current  reverses  with  the  flux.  Currents  will  be  induced  in  the  coil  if  it  remains 
stationary  and  the  magnetic  flux  passes  in  and  out  of  the  coil,  or  if  the  field  is  stationary 
and  the  coil  is  moved  through  it.  In  either  case,  an  E.  M.  F.  is  generated  proportional  to 
the  rate  at  which  the  conductor  cuts  through  the  field. 

We  may  substitute  for  the  bar  magnet  just  mentioned  a  solenoid  winding  P  and  cause 
its  magnetic  field  to  act  upon  a  second  winding  S  as  in  Fig.  19.  When  the  circuit  of  wind- 
ing P  is  opened  at  key  K,  no  lines  of  force  are  in  evidence,  but  at  the  moment  the  key  is 
closed,  the  lines  of  force  expand  from  the  core  P  and  intersect  or  cut  through  the  winding 
S.  The  galvanometer  then  gives  a  momentary  deflection.  If  the  current  is  left  to  flow 
through  P,  there  is  no  further  effect  in  S  until  the  circuit  of  P  is  opened  by  the  key;  the 
galvanometer  now  gives  a  second  momentary  deflection  but  the  needle  moves  in  the  opposite 
direction  just  as  in  the  case  of  the  bar  magnet.  Thus  for  each  "make"  and  "break"  of  the 
first  circuit,  two  pulses  of  current  flow  through  the  winding  S,  the  first  in  one  direction 
around  the  circuit,  and  the  second  in  the  op- 
posite direction.  This  current  is  said  to  be 
induced  in  S  by  electromagnetic  induction. 

29.  Mutual    Induction. — It    is    of 

great  importance  to  note  that  the  effect  in 
S  takes  place  only  when  the  circuit  of  P 
is  made  and  broken.  When  current  is 
flowing  in  P  continuously,  the  magnetic 
lines  of  force  are  stationary,  and  conse-  /  i- 


t  ,'"' 

I    I 


quently  current  is  not  induced  in  S.  But 
when  the  lines  of  force  about  S  rise  and  \ 
fall,  then  there  will  be  a  movement  of  cur- 
rent through  S.  If  the  winding  S  is  placed 
at  a  right  angle  to  P  instead  of  lying  paral- 
lel to  it,  a  change  of  flux  in  P  will  have 
little  or  no  effect  upon  S. 


V 


Fig.    15 — Magnetic   Field   of   Solenoid   Winding. 


20 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Fig.    16 — Magnetic    Field   of  Horse-shoe   Magnet   with 
Current    Flowing    in    Definite    Directions. 


HIM 


Fig.     17 — Showing     How     Magnetic     Field     Reverses 
with   Reversal  of  Current. 


The  direction  of  the  current  induced  in 
S  should  be  observed.  When  the  lines  of 
force  increase  through  S,  the  induced  pres- 
sure is  counter  to  that  which  originally 
flowed  in  winding  P,  but  when  the  lines 
of  force  decrease  through  S,  the  induced 
current  has  the  same  direction  as  the  orig- 
inal current  from  the  battery  through 
winding  P. 

It  is  clear  that  the  lines  of  force  in  S 
are  in  the  opposite  direction  to  those  which 
set  up  the  current  in  S.  The  field  of  force 
created  around  S  therefore  reacts  upon 
winding  P  tending  to  build  up  a  current 
in  opposition  to  that  already  flowing  in  P. 
That  is,  the  change  in  strength  of  the 
primary  current  in  P  induces  a  secondary 
current  in  S  which  in  turn  induces  a  back 
pressure  in  P.  The  induction  due  to  the 
two  circuits  reacting  upon  each  other 
is  called  their  mutual  induction  which  is  a 
measurable  quantity. 

30.  Self-induction.  —  We  have 
seen  that  the  expanding  field  of  wind- 
ing P  induces  an  electromotive  force 
in  winding  S.  Similarly  the  field 
produced  by  each  turn  in  winding  P 
will  cut  neighboring  turns,  thereby  in- 
ducing in  them  electromotive  forces 
that  tend  to  oppose  the  E.  M.  F.  of 
the  original  current.  On  the  other 
hand,  when  the  current  in  winding  P 
diminishes,  the  lines  of  force  contract 
and  thereby  induce  electromotive 
forces  in  adjacent  turns,  that  tend  to 
set  up  currents  in  the  same  direction 
as  the  original  current. 

This  inductive  action  of  a  coil  or 
conductor  upon  itself  is  called  self- 
induction. 

Self-induction  may  be  defined  as 
the  property  of  a  circuit  that  tends  to 
prevent  any  change  in  the  strength  of 
currQnt  passing  through  it.  This  is 
clear  from  the  fact  that  self-induced 
currents  either  tend  to  prevent  the 
rise  or  the  fall  of  current  through  a 
circuit. 

The  effects  of  self-induction  are 
noticeable  only  in  direct  current  cir- 
cuits when  the  current  is  turned  on 
and  off,  but  in  alternating  current  cir- 
cuits they  are  ever  present.  All  con- 
ductors have  self-induction.  the 
amount  depending  upon  their  size  and 
shape.  Coiled  wires  have  greater 
self-induction  than  a  long  straight 
wire.  The  self-induction  of  a  coil 


ELECTROMAGNETIC   INDUCTION. 


21 


without  an  iron  core  is  practically 
constant.  If  a  given  coil  has  an  iron 
core,  the  self-induction  is  greater  in 
proportion  to  the  permeability*  of  the 
iron. 

The  coefficient  of  self-induction  or 
inductance  is  also  defined  as  the  prop- 
erty of  a  conductor  by  which  energy 
may  be  stored  up  in  magnetic  form. 

The  unit  of  inductance  is  the 
henry  and  represents  the  cutting  of  100,000,000  lines  of  force  when  one  ampere 
of  current  is  turned  on  and  off  per  second ;  that  is,  if  one  ampere  is  turned  on  and 
off,  in  a  given  conductor,  the  electromotive  force  induced  by  the  collapse  of  the 
magnetic  field,  equals  one  volt. 


Fig. 


k 


18 — Variation    of    Magnetic    Field    by    Opposing 
Coils. 


Fig.    19 — Diagram   Illustrating  the  Principle  of  Electromagnetic  Induction. 

This  means  that  a  conductor  or  coil  to  have  self-induction  of  one  henry  must  be  of  such 
length  and  shape  that  when  one  ampere  is  flowing  it  is  surrounded  by  100,000,000  lines  of 
force,  and  when  the  current  is  turned  on  and  off,  100,000,000  lines  of  force  cut  through  the 
conductor  setting  up  a  pressure  of  one  volt. 

This  can  be  expressed : 

M  X  T 


L- 


I  X  100,000,000 
where  T  =  the  total  number  of  turns  in  a  given  coil; 

M  =  the  total  lines  of  force  threading  through  the  coil ; 
and  I  =  the  current  in  amperes. 

If  M  =  the  lines  of  force  threading  through  the  coil  when  the  current  =  1  ampere,  then 

M  X  T 


L  = 


100,000,000 

The  unit,  the  henry,  is  applicable  to  coils  of  a  great  numlDer  of  turns  having  iron  cores,  but 
for  coils  encountered  in  wireless  telegraph  transmitters,  some  sub-multiple  of  the  henry  is 
desirable,  such  as  the  micro-henry,  the  milli-henry  and  the  centimeter. 

*  Permeability  is  a  measure  cf  the  ability  of  a  magnetic   substance  to  conduct  magnetic  lines  of  force. 


22 


PRACTICAL  WIRELESS  TELEGRAPHY. 


1,000  centimeters  =  1  micro-henry; 

1 
1  micro-henry  =.—  —henry; 


1,000,000 
1 


1  rmlli-henry  — 


henry; 


1,000 

The  inductance  of  a  given  circuit  is  generally  calculated  by  one  of  several  formulae. 
31.  Value  of  Induced  E.  M.  F. — Referring-  to  Fig.  19:  The  electromotive 
force  induced  in  winding  S  is  conditioned  on  the  ratio  of  the  turns  in  the  two  windings 
and  the  rate  of  the  change  of  flux  threading  through  S.  For  instance,  if  P  has  100  turns 
of  comparatively  coarse  wire  such  as  No.  14  or  No.  16  B.  &  S.  wound  over  an  iron  core 
and  S  has  many  thousand  turns  of  fine  wire  such  as  No.  36  B.  &  S.,  an  electromotive  force 
of  several  hundred  thousand  volts  may  be  induced  in  S.  Should  winding  S  have  less  turns 
than  winding  P,  the  E.  M.  F.  induced  in  S  will  be  lower  than  that  of  winding  P.  Ad- 
vantage of  this  principle  is  taken  in  the  design  of  the  apparatus  known  as  the  induction 
coil,  in  which  the  circuit  of  P  is  interrupted  from  thirty  to  one  hundred  times  per  second. 
Then  if  winding  S  is  given  a  large  number  of  turns  and  its  terminals  are  separated  by  a 
space  of  several  inches,  the  voltage  may  be  so  great  as  to  jump  the  gap  in  the  form  of  an 
electric  spark. 

To  properly  distinguish  the  various  circuits,  winding  P  is  called  the  primary;  winding  S 
the  secondary  winding.  The  current  in  P  is  termed  the  primary  current  and  in  S  the 
secondary  current. 


Fig.  20 — Fundamental  Diagram  of  Simple  Alternator. 

32.  The  Dynamo. — We  may  briefly  define  the  dynamo  as  a  machine  for 
converting  mechanical  energy  into  electrical  energy  by  the  principle  of  electro- 
magnetic induction.  But  unlike  the  simple  battery  or  storage  cell  the  dynamo 
may  generate  either  direct  or  alternating  current.  Alternating  current  dynamos 
are  frequently  called  alternators.  The  student  can  nearly  always  distinguish 
between  the  two  machines  by  observing  the  part  of  the  dynamo  at, which  the 
current  is  collected.  If  the  brushes  rest  on  a  commutator  made  up  of  a  number 
of  copper  segments  separated  by  insulating  material,  it  will  be  a  direct  current 
dynamo,  but  if  the  brushes  simply  rest  on  two  brass  rings,  it  will  be  an  alternating 
current  dynamo. 

The  fundamental  principle  of  the  dynamo  follows :  Whenever  a  coil  of  wire 
rotates  through  a  magnetic  field  of  uniform  strength  in  such  a  way  that  the 


ELECTROMAGNETIC   INDUCTION. 


23 


number  of  lines  of  force  enclosed  by  the  coil  increase  or  diminish  uniformly,  a 
current  of  electricity  will  be  induced  in  the  coil,  the  strength  of  which  at  any 
instant  is  proportional  to  the  rate  of  the  change  of  flux. 
Hence  the  essentials  of  a  dynamo  are : 

(1)  A  magnetic  field  of  constant  strength; 

(2)  A  number  of  coils  mounted  on  a  shaft  and  rotated  in  such  a  way  as  to  cut 
through  the  magnetic  field; 

(3)  Means  for  conducting  the  current  induced  in  the  rotating  coils  to  an  outside 
circuit. 

A  diagram  of  an  elementary  alternator  appears  in  Fig.  20.  A  uniform  magnetic  field  is 
set  up  between  the  magnetic  poles  N  and  S  by  the  current  from  battery  B-l  which  flows 
through  the  magnet  windings  M,  M.  The  rectangle  of  wire  A,  B  is  mounted  on  a  shaft 
which  rotates  clockwise.  Two  brass  rings,  C,  D,  are  mounted  on  the  shaft  but  insulated 
from  it.  The  copper  brushes  H  and  L  make  contact  with  these  rings,  and  the  circuit  is 
completed  through  F  (any  current  absorbing  apparatus). 

According  to  the  principle  just  explained,  if  A,  B  rotates  around  its  axis,  an  E.  M.  F. 
will  be  induced  in  the  loop,  the  magnitude  of  which  depends  on  the  rate  of  charge  of  the 
number  of  lines  of  force  threading  through  the  loop.  When  in  the  vertical  position  of  Fig. 
20,  the  loop  encloses  the  maximum  number  of  lines  of  force,  but  when  side  A  goes  under- 
neath the  S  pole  and  side  B  goes  under- 
neath the  end  pole,  as  in  Fig.  21,  the  rect- 
angle will  enclose  the  minimum  number  of 
lines  of  force  when  it  has  moved  90  de- 
grees or  in  a  horizontal  position.  As  A 
moves  out  of  the  field  of  the  south  pole 
and  B  out  of  the  field  of  the  north  pole, 
the  rectangle  reaches  another  vertical  posi- 
tion (but  with  the  two  sides  of  the  rect- 
angle reversed)  and  again  encloses  the 
maximum  number  of  lines  of  force.  As 
the  rotation  of  A,  B  continues,  side  A  -goes 
into  the  field  of  the  N  pole  and  side  B 
goes  into  the  field  of  the  S  pole,  where 
for  a  second  time  the  minimum  number 
of  lines  of  force  are  enclosed  after  which 
the  loop  returns  to  the  position  mentioned 
at  the  beginning. 

Now,  according  to  the  rule  which  gov- 
erns the  direction  of  the  flow  of  current  in 
a  conductor  cutting  through  a  magnetic 
field,  when  A,  B,  is  in  the  position  of  Fig. 


Fig.  21 — Showing  Position  of  Armature  Conductors  for 
Maximum  Cutting. 


21,  a  current  will  flow  towards  the  rear  of  the  rectangle  in  the  left  hand  side,  and  towards 
the  front  of  the  rectangle  on  the  right  hand  side.  Then  if  A,  B,  continues  y2  revolution, 
so  that  side  A  is  cutting  through  the  N  field  and  side  B  through  the  S*  field,  current  will 
flow  in  A,  B,  in  the  opposite  direction.  It  is  clear  that  in  a  complete  revolution,  A,  B, 
undergoes  two  changes  of  current  which  flows  first  in  one  direction  around  the  rectangle 
and  then  in  the  opposite  direction.  The  current  is  said  to  have  gone  through  a  complete 
cycle. 

We  see  that  during  the  first  quarter  revolution  of  loop  A,  B,  or  from  0°  to  90°,  the 
E.  M.  F.  increases  from  zero  to  maximum;  from  90°  to  180°  the  E.  M.  F.  decreases  from 
maximum  to  zero;  from  180°  to  270°  the  current  reverses  and  the  E.  M.  F.  increases  from 
zero  to  maximum,  and  from  270°  to  360°  the  E.  M.  F.  again  decreases  from  maximum  to 
zero. 

The  changes  in  the  strength  of  the  current  induced  in  A,  B,  can  be  shown  by  a  wave-like 
curve  as  in  Fig.  22,  in  which  the  successive  positions  of  the  rectangle  are  shown  by  the 
positions,  1,  2,  3,  4,  5,  6,  7,  8,  etc.  From  position  8-16  the  E.  M.  F.  gradually  rises,  maximum 
E.  M.  F.  being  attained  in  position  4-12.  This  increase  of  E.  M.  F.  is  indicated  by  the 
ascending  slope  of  the  curve  B  to  C.  From  position  4-12  on,  the  E.  M.  F.  decreases  (as 
indicated  by  the  descending  slope  of  the  curve  B  to  C),  the  minimum  cutting  of  the  lines 
of  force  taking  place  at  point  8-16.  This  corresponds  to  the  point  X1  on  the  horizontal  line. 
As  the  rectangle  continues  the  revolution,  the  lines  of  force  are  cut  on  an  increasing  angle, 
another  maximum  of  E.  M.  F.  being  attained  at  point  12-4,  but  of  the  opposite  sign  as 


24 


PRACTICAL  WIRELESS  TELEGRAPHY. 


shown  at  point  D.  From,  this  point  the  E.  M.  F.  decreases  to  zero  or  when  the  rectangle 
A,  B,  is  in  the  position  of  16-8.  This  curve  depicts  the  gradual  rise  and  fall  of  the  E.  M.  F. 
in  a  dynamo  coil  and  is  known  as  a  sine  curve,  the  plotting  of  which  is  explained  in  the 
principles  of  co-ordinate  geometry.  The  curve  shows  the  relation  between  time  (fractions 
of  a  second)  and  the  strength  or  amplitude  of  the  current  at  any  given  point  during  the 
complete  revolution  of  a  dynamo  coil.  The  curve  represents  a  complete  cycle  of  alternating 
current.  Vertical  lines  drawn  from  the  horizontal  A,  B,  represent  time  in  fractions  of  a 
second.  The  horizontal  lines  drawn  from  the  successive  positions  of  the  coil,  1,  2,  3,  4,  etc., 
correspond  to  the  position  of  the  dynamo  coil  at  any  particular  instant.  At  points  where 
the  horizontal  and  vertical  lines  intersect,  a  common  line  is  drawn  connecting  them,  which 
results  in  the  wave-like  curve. 

33.  Determination  of  Frequency. — The  frequency  of  an  alternating  cur- 
rent dynamo  is  expressed  in  cycles  per  second.  We  see  from  the  previous  para- 
graph that  one  complete  cycle  of  current  is  generated  when  A,  B,  makes  a  single 
revolution.  Hence  if  A,  B,  rotates  60  complete  revolutions  per  second,  there  will 
be  120  reversals  or  alternations  of  current  per  second.  Since  two  alternations  of 


IN  CASE  OF  500  CYCLE  ALTERNATOR 
Fig.  22 — Sine  Curve  Showing  Rise  and  Fall  of  Current  during  One  Complete  Cycle. 

current  constitute  a  complete  cycle,  the  frequency  of  this  generator  is  said  to  be  60 
cycles. 

The  frequency  of  any  alternator  may  be  determined  by  first  counting  the 
number  of  field  poles  and  by  measuring  the  speed  of  the  armature  per  second  of 
time,  or 

N  X  S 

Frequency  = 

v  2 

Where  N  =  the  number  of  field  poles ; 

S  =  the  speed  of  the  armature  in  revolutions  per  second. 

Direct  reading  frequency  meters  are  in  daily  use.  They  are  connected  in 
shunt  to  the  power  circuit  like  the  voltmeter.  (Frequency  meter  is  described  in 
Paragraph  49,  section  C.) 

34.  Strength  of  Magnetic  Field. — The  strength  of  the  magnetic  field  about 
the  poles  N  and  S,  Fig.  20,  is  proportional  to  the  strength  of  the  current  in  amperes  and  the 
number  of  turns  of  the  coil.  The  strength  of  the  magnetic  field  Is  the  same  whether  a 
current  of  a  large  number  of  amperes  is  flowing  through  a  few  turns  6f  wire  or  a  relatively 
weak  current  flows  through  a  greater  number  of  turns.  The  turns  of  the  field  winding 
of  any  dynamo  are  of  a  fixed  number ;  hence  the  strength  of  the  magnetic  field  is  regulated 
by  increase  or  decrease  in  the  strength  of  the  current  flowing  through  tthe  field  winding. 


ELECTROMAGNETIC   INDUCTION. 


25 


The  field   current   is   regulated  by  a   device  known   as   a   field  rheostat   which   is   simply  a 
variable  resistance  connected  in  series  with  the  circuit. 

The  voltage  developed  in  any  given  dynamo  coil  is  proportional  to  the  rate  of  cutting 
of  the  magnetic  field.  In  the  case  of  the  rectangle  A,  B,  of  Fig.  20  or  21,  the  total  flux 
passes  in  the  coil  twice  and  out  twice  during  one  revolution  or  during  one  cycle.  If  the 
coil  enclosed  100,000,000  lines  of  force  and  made  one  complete  revolution  per  second, 
100,000,000  lines  of  force  would  be  thrust  into  the  coil  twice  and  thrust  out  twice.  This 
would  be  the  equivalent  of  cutting  400,000,'OOQ  lines  of  force  per  second  and  in  this 
particular  case  the  induced  E.  M.  F.  would  be  4  volts.  If  the  number  of  turns  on  the 
armature  winding  were  doubled,  all  other  conditions  remaining  equal,  the  voltage  would 


1 10  VOLT  D.C. 


FIELD  POLE 


IIOVOLTS 

60-500 

CYCLES 


-O- 
-O- 


LQAD 

Fig.  23 — Fundamental   Diagram  of  4  Pole  A.   C.   Generator. 

be  double,  or  we  might  state  that  if  there  were  N  turns,  the  E.  M.  F.  developed  would 
be  N  X  that  of  1  turn. 

The  fundamental  equation  for  the  dynamo  is  : 

4XNXnXS 


E  — 


100,000,000  X  P 
Where  E  =  the  average  voltage  of  the  dynamo ; 

n  =  the  revolutions  of  the  armature  per  second  ; 
N  —  the  number  of  conductors  on  the  surface  of  the  armature  ; 
P  =  the  number  of  pairs  of  poles  ; 
S  =  the  total  number  of  lines  of  force. 
In  a  commercial  dynamo,  the  only  factors  in  this  equation  which  are  variable, 


26 


PRACTICAL  WIRELESS  TELEGRAPHY. 


are  (1)  the  density  of  the  magnetic  field  and  (2)  the  speed  of  the  dynamo  arma- 
ture per  second.  We  see  from  this  that  the  voltage  of  any  generator  may  be 
increased  by  increase  of  the  speed  of  the  armature  or  by  increase  of  the  strength 
of  the  magnetic  field  surrounding  the  armature  coils.  Commercial  generators  are 
built  usually  for  constant  speed.  Regulation  of  the  voltage  is  obtained  by  means 
of  the  field  rheostat. 

35.  Diagram  of  an  Alternating  Current  Dynamo. — The  essential  parts  of 
an  alterating  current  dynamo  are : 

(1)  Field  Magnets. 

(2)  Armature. 

(3)  Collector  Rings. 

The  diagram  of  Fig.  23  is  merely  intended  to  show  the  general  details  of  the 
construction  and  connections  of  an  alternating  current  dynamo.  The  field  poles 
which  are  firmly  bolted  to  the  circular  iron  frame  are  represented  by  N,  S,  N,  S, 
the  armature  at  M  and  the  collector  rings  at  E,  F.  The  field  poles  are  wound 
alternately  in  opposite  directions  so  that  the  current  circulates  about  the  turns  in 
opposite  directions,  giving  the  poles  alternatively  north  and  south  polarity.  The 
armature  M  is  built  up  of  a  number  of  slotted  sheets  of  soft  iron  which  are  pinned 
together  and  mounted  on  a  common  shaft,  the  copper  conductors  lying  lengthwise 
of  the  core  in  such  a  way  that  the  coils  will  be  filled  and  emptied  with  magnetic 
flux  (coils  not  shown).  If  these  coils  are  properly  connected  together,  the 
currents  induced  therein  (by  the  change  of  flux)  will  flow  in  the  same  gen- 
eral direction,  the  voltage  of  one  coil  being  added  on  to  that  of  the  next  coil. 
It  is  to  be  especially  noted  that  the  source  of  continuous  or  direct  current  for 
exciting  the  field  poles  of  an  alternator  is  generally  supplied  from  an  external 
source  which  may  be  either  a  small  direct  current  dynamo  known  as  an  exciter 
or  a  battery  of  storage  cells.  In  most  cases  encountered  in  wireless  working,  the 
pressure  of  the  direct  current  source  is  110  volts  and  the  number  of  the  turns  of 
the  field  winding  are  such  that  the  correct  amount  of  current  flows  with  small 
amounts  of  resistance  in  series  at  the  rheostat  R.  As  already  explained,  this 
resistance  is  known  as  the  field  rheostat  or  field  regulator. 

When  the  armature  M  revolves  at  a  uniform  rate,  an  alternating  current  is  induced  in 
the  coils  which  is  collected  by  the  brushes  E,  F  in  contact  with  2  collector  rings,  the  volt- 
tage  varying  with  the  design  of  the  machine.  For  purposes  of  wireless  telegraphy  the 
voltage  of  the  generator  may  vary  from  110  to  500  volts  and  the  frequency  of  the  current 
may  vary  from  60  to  500  cycles  standard  frequencies  being  60,  120,  240  and  500  cycles. 

If  the  armature  of  Fig.  23  were  revolved  1800  revolutions  per  minute,  current  at  a 
frequency  of  60  cycles  per  second  would  be  obtained  from  its  armature.  Remembering  the 
formula  given  for  determining  the  frequency  we  see  that  in  a  complete  revolution  of  the 
armature  any  point  passes  through  four  fields  which  would  set  up  four  reversals  of  current. 

If  the  armature  revolves  at  the  rate  of 
1800  revolutions  per  minute,  corresponding 
to  30  revolutions  per  second,  there  would 
be  a  4  X  30  or  120  reversals  of  current, 
or  a  frequency  of  60  cycles.  If  the  genera- 
tor had  32  field  poles,  the  frequency  would 
be  32  X  30  or  960  alternations  correspond- 
ing to  480  cycles. 

The  student  should  understand  that  the 
foregoing  description  and  drawing  simply 
shows  in  an  elementary  way  the  construc- 
tion and  functioning  of  a  generator.  The 
diagram  is  merely  intended  to  indicate  the 
connections  of  the  machine,  the  direction 
of  the  magnetic  lines  of  force  and  the 
method  by  which  the  voltage  generated  by 
Fig.  24.— Showing  the  Function  of  a  Simple  Commutator,  the  armature  is  regulated. 


ELECTROMAGNETIC   INDUCTION. 


27 


36.  Direct   Current   Dynamo. —  Direct  current  is  obtained  from  dynamo  coils 
by  a  commutator,  which  is  placed  on  one  end  of  the  armature  driving  shaft.     In 
simple  form  it  consists  of  a  split  brass  or  copper  ring  of  two  parts,  C,  D,  which 
is  thoroughly  insulated  from  the  armature  shaft  (shown  in  Fig.  24).   The  circuit 
from  the  loop  A,  B  is  completed  through  the  contact  brushes  E  and  F  through 
an  external  load  as  at  R. 

The  function  of  the  commutator  should  be  clear  from  the  following  explanation  :  As- 
sume the  coil  A,  B  to  be  in  rotation  in  the  direction  of  the  arrow ;  then  in  the  particular 
position  shown  in  Fig.  24,  the  segment  D  will  be  a  (-f )  pole  and  segment  E  a  ( — )  pole. 
The  current  will  therefore  flow  in  the  external  circuit  from  brush  F  to  brush  E.  When 
A,  B  turns  completely  over  so  that  side  B  goes  under  the  south  pole  and  side  E  under  the 
north  pole,  the  current  will  flow  in  B  as  it  did  formerly  in  side  A,  that  is,  towards  the 
brush  F.  Similarly,  when  A  is  in  the  north  field,  the  current  will  flow  away  from  brush 
E.  Therefore,  the  current  will  flow  in  the  external  circuit  in  the  same  direction  as  in  the 
first  case. 

We  see  also  that  in  the  second  position  mentioned,  the  current  in  B  is  flowing  oppositely 
to  that  when  B  was  cutting  through  the  north  field,  but  we  must  keep  in  mind  that  com- 
mutator segment  B  now  makes  contact  with  brush  F  instead  of  brush  E.  Thus  the  cur- 
rent will  flow  in  one  direction  in  the  external  circuit  irrespective  of  the  rate  at  which  A,  B 
revolves. 

A  steady  flow  of  current  like  that  obtained  from  a  battery  of  chemical  cells  cannot  be 
obtained  from  the  dynamo;  the  latter  in  reality  generates  a  pulsating  current.  If  the  dynamo 
armature  is  composed  of  a  great  number  of  coils,  the  pulsations  are  so  minute  and  follow 
each  other  so  rapidly  that  the  current  is  practically  continuous.  That  is,  these  pulsations 
are  made  to  overlap  one  another  by  mounting  a  number  of  loops  of  the  armature  and 
connecting  them  in  series  so  that  immediately  one  set  of  coils  passes  the  position  of 
maximum  cutting  of  the  lines  of  force,  another  set  will  take  their  place.  The  greater  the 
number  of  the  armature  coils  the  greater  will  be  the  number  of  commutator  segments 
required.  In  fact,  commutators  in  commercial  dynamos  may  have  from  50  to  150  segments 
depending  upon  the  design  of  the  dynamo. 

37.  Shunt,   Series,   and  Compound  Wound   Dynamos. — We   have  already 
explained  that  continuous  or  direct  current  must  flow  through  the  field  windings 
of  an  alternating  current   dynamo   and  that  this   current  is  obtained   from  an 
external  source.     In  the  direct  current  dynamo,  the  current  for  excitation  of  the 
field  is  obtained  from  its  own  armature. 

When  the  terminals  of  the  field  winding  are  tapped  across  the  brushes  of  a  direct  cur- 
rent dynamo,  it  is  called  a  shunt  wound  dynamo.  The  circuit  for  this  machine  is  shown  in 
the  diagram,  Fig.  25,  where  the  terminals  of  the  field  winding  are  tapped  across  the  arma- 
ture circuit  at  points  C  and  D.  A  regulating  rheostat  connected  in  series  with  the  field 
circuit  at  R  permits  an  increase  or  decrease  of  the  strength  of  the  current  flowing.  The 
field  winding  of  the  shunt  dynamo  is  composed  of  a  large  number  of  turns  of  com- 
paratively fine  insulated  wire,  the  actual 
number  of  turns  being  governed  by  the 
flux  required,  whereas  the  armature  coils 
have  comparatively  coarse  wire.  Two 
paths  are  presented  to  the  current  as  it 
flows  from  the  armature  of  this  machine, 
one  being  the  field  circuit  and  the  other, 
the  external  circuit. 

In  well  designed  shunt  dynamos,  the  re- 
sistance of  the  shunt  circuit  is  always 
greater  than  the  resistance  of  the  armature 
and  external  circuit,  but  the  strength  of  the 
current  flowing  in  the  shunt  coil  is  in  fact 
comparatively  small  even  in  the  larger  types 
of  generators. 

The  student  may  question  how  current 
is  set  up  in  a  machine  of  this  type  when  it 
is  first  put  into  motion.  The  fact  is  that  the 
initial  building  of  the  current  is  due  to 
residual  magnetism  in  the  field  cores.  When  Fl>.  25-Circuit  of  Shunt  Wound  D.  C.  Generator. 


28 


PRACTICAL  WIRELESS  TELEGRAPHY. 


FIELD 


ARMATURE 


a  piece  of  soft  iron  has  been  magnetized,  no  matter  how  soft  the  iron  may  be,  a  certain  num- 
ber of  magnetic  lines  of  force  are  retained  when  the  magnetizing  current  has  been  turned 
off.  These  lines  are  known  as  the  residual  lines  of  force  and  the  cores  of  the  field  winding 
are  said  to  possess  residual  magnetism. 

When  the  dynamo  armature  is  first  set  into  rotation,  the  residual  lines  of  force  pass  in 
and  out  of  the  armature  conductors  through  the  core,  generating  therein  a  feeble  current 
which  flows  to  the  field  winding  and  increases  the  number  of  lines  of  force  threading  through 
the  armature  coils.  This  induces  a  stronger  current  in  the  armature  conductors  which  con- 
tinually adds  to  the  strength  of  field  until  the  normal  voltage  of  the  dynamo  is  established. 
The  complete  process  usually  requires  from  10  to  50  seconds.  After  the  generator  armature 
attains  its  normal  speed,  the  voltage  across  its  terminals  may  be  raised  or  lowered  by  the 
rheostat  R.  If  the  resistance  of  R  is  increased,  the  voltage  diminishes,  or  if  the  resistance 
of  R  is  decreased,  the  voltage  increases. 

A  diagram  of  a  series  wound  generator  appears  in  Fig.  26.  The  field  magnets 
of  this  type  are  wound  with  a  few  turns  of  thick  wire  joined  in  series  with  the 

armature  brushes  and  all  of  the  cur- 
rent generated  by  the  armature  passes 
through  the  coils  of  the  field  magnet 
to  the  external  circuit.  The  current  in 
passing  through  the  windings  of  the 
field  magnet,  energizes  them  and 
strengthens  the  weak  field  due  to  the 
residual  magnetism  of  the  cores  which 
results  in  a  gradual  building  up  of  the 
magnetic  field.  The  important  char- 
acteristic of  this  machine  is  its  ability 
to  furnish  current  at  increased  voltage 
as  the  load  increases  for  it  is  clear  from 
previous  explanations  that  the  greater 
the  strength  of  the  field  current,  the 
greater  the  strength  of  the  magnetic 
field  from  pole  to  pple.  The  strength 
of  the  field  current  flowing  through  a 
series  wound  generator,  and  therefore 
the  voltage  across  its  armature  is  reg- 
ulated by  cutting  out  turns  of  the  field 
lerator.  through  the  medium  of  a  multi-point 

switch  or  as  may  be  done  in  the  case  of  any  type  of  generator,  the  voltage  can  be 
regulated  by  variation  of  the  speed  of  the  armature. 

The  compound  wound  dynamo  combines  the  desirable  characteristics  of  both 
the  series  and  shunt  wound  machine,  and  it  gives  a  better  regulation  of  voltage 
on  circuits  of  varying  load  than  is  possible  with  a  dynamo  of  either  type.  A 
suitable  diagram  of  connections  appears  in  Fig.  27.  The  field  magnets  of  the 
compound  dynamo  are  wound  with  two  sets  of  coils,  one  set  being  connected  in 
series  with  the  armature  as  shown  at  R,  and  another  set  in  shunt  to  the  armature 
and  external  circuit  as  shown  at  V.  The  function  of  the  series  winding  is  to 
strengthen  the  magnetic  field  by  the  current  taken  through  the  external  circuit, 
and  thus  automatically  sustain  the  voltage  under  variation  of  a  load. 

In  the  case  of  the  shunt  wound  dynamo,  as  the  external  load  is  increased,  the  potential 
difference  at  the  armature  terminals  will  fall,  but  in  the  case  of  the  compound  wound  gen- 
erator, this  fall  of  pressure  is  counteracted  by  the  series  winding,  the  current  which  flows 
in  it  increasing  with  the  load  and  causing  the  pressure  to  rise.  The  number  of  turns  of  each 
winding  and  the  relative  strength  of  current  is  proportioned  so  that  a  practically  constant 
pressure  is  maintained  under  varying  load.  Initial  adjustments  of  the  voltage  can  of  course 
be  secured  by  means  of  a  field  rheostat  such  as  shown  at  R-l. 

The  student  should  note  carefully  that  current  must  circulate  in  both  the 


ELECTROMAGNETIC   INDUCTION. 


29 


series  and  shunt  windings  in  the  same  general  direction  in  order  that  the  re- 
sultant magnetic  fields  may  have  the  same  general  direction. 

38.  The  Electric  Motor.— A  motor  is  a  machine  for  converting  electrical 
energy  into  mechanical  energy.  There  is  essentially  no  difference  between  a 
motor  and  a  dynamo.  Any  dynamo  connected  to  a  source  of  electric  power  will 
run  as  a  motor  and  any  motor  driven  by  mechanical  power  such  as  a  steam 
engine,  etc.,  will  generate  a  current  of  electricity.  The  differences  between  the 
two  machines  are  mainly  mechanical. 
The  fundamental  operating  principle  of 
the  motor  is  as  follows:  A  wire  carrying 
a  current  placed  in  a  magnetic  field  will 
tend  to  move  in  a  direction  at  right  angles 
both  to  the  direction  of  the  field  and  to  the 
direction  of  the  current.  For  example,  if 
the  plane  of  a  given  coil  of  wire  lying  be- 
tween the  poles  of  a  magnet  is  parallel  to 
a  magnetic  field,  and  a  current  is  passed 
through  the  coil,  it  will  tend  to  turn  or 
to  take  up  a  position  at  a  right  angle  to  the 
magnetic  field.  If  the  current  is  reversed 
when  it  has  reached  this  position,  the  coil 
will  continue  to  revolve. 

The  action  of  the  motor  can  be  simply 
explained  by  the  diagram  of  Fig.  28  where 
a  motor  armature,  commutator  and  brushes 
as  well  as  the  field  poles,  are  represented 
in  a  conventional  manner.  If  the  terminals 
G,  H,  be  connected  to  a  source  of  direct 
current,  part  of  the  current  will  circulate 
through  the  field  windings  and  part  through 
the  coils  of  the  armature  between  the  two 

brushes.  If  the  current  flowing  through  Fig.  27— Circuit  of  Compound  Wound  D.  C.  Generator, 
the  armature  coils  bears  the  correct  direction  to  that  flowing  through  the  field  winding,  a 
state  of  magnetism  such  as  shown  in  Fig.  28  may  be  produced.  The  upper  half  of  the  arma- 
ture core  above  the  imaginary  line  X,  will  be  a  south  pole  and  the  lower  half  a  north  pole. 
The  lower  half  of  the  armature  will  then  be  attracted  by  the  south  field  pole  and  repelled  by 
the  north  field  pole  and  the  upper  half  will  be  repelled  by  the  south  field  pole  and  attracted 
by  the  north  field  pole.  This  may  be  stated  in  another  way  by  stating  that  the  coils  of  the 
armature  tend  to  turn  until  they  enclose  the  greatest  number  of  lines  of  force  from  the  field 
poles.  The  general  strain  of  this  attraction  and  repulsion  is  seen  to  be  clockwise. 

The  movement  of  the  armature  will  be  continuous  because  the  commutator  acts  to  main- 
tain in  the  same  direction  the  flow  of  current  through  the  two  sides  of  the  armature  always. 
Consequently,  the  upper  half  of  the  armature  will  always  be  a  south  pole  while  the  lower 
half  will  be  a  north  pole,  irrespective  of  the  speed  at  which  the  armature  revolves. 

Now  it  would  have  no  effect  on  the  general  direction  of  rotation  if  the  connections  from 

the  source  of  current  to  the  motor  were 
reversed  because  the  polarity  of  the  flux 
in  both  the  armature  and  the  field  poles 
would  be  reversed  accordingly  and  be- 
cause the  strain  of  the  two  magnetic  fields 
would  have  the  same  general  direction 
the  motor  would  revolve  in  the  same  di- 
rection as  before.  Careful  consideration 
of  this  fact  reveals  that  in  order  to 
change  the  direction  of  rotation  of  the 
armature,  the  flow  of  current  must  be 
reversed  independently  in  either  the 
armature  or  field  poles. 

Like    generators,     motors    may    have 
series,   shunt,  or  compound  wind- 


110  VOLT  O.C. 


Fi6' 


d 


30 


PRACTICAL  WIRELESS  TELEGRAPHY. 


ings.     The  type  known  as  the  differential  wound  motor  appears  in   Fig.  29  and  will   be 
described  further  on. 

39.  The  Effect  of  Counter  Electromotive  Force. — When  a  motor  armature 
is  set  into  motion  by  an  external  current,  the  loops  of  wire  composing  its  coils  cut  through 
the  magnetic  field  and  induce  a  reverse  electromotive  force  counter  to  that  which  originally 
caused  the  motion.  This  back  pressure  is  known  as  counter  electromotive  force  which  gov- 
erns directly  the  speed  of  a  motor.  The  difference  between  the  impressed  and  the  counter 
voltage  determines  the  actual  flow  of  current  in  the  armature  and  the  counter  voltage  is 
proportional  to  the  speed  of  the  armature,  the  number  of  armature  wires  and  the  strength 
of  the  magnetic  field  which  is  enclosed. 

The  speed  of  a  motor  supplied  with  current  at  constant  pressure  varies  directly  with  the 
counter  electromotive  force  and  in  any  given  machine  the  stronger  the  field,  the  slower  will 

be  the  speed  of  the  armature.  If  the  field 
of  a  motor  be  weakened  by  inserting  re- 
sistance in  the  excitation  circuit,  the 
armature  will  increase  its  speed  up  to  a 
certain  point,  or  until  the  increased  speed 
of  the  armature  increases  the  counter 
E.  M.  F.  to  such  an  extent  as  to  cut  down 
the  armature  current.  Up  to  this  point, 
however,  the  speed  of  any  given  motor 
can  be  varied  by  simply  increasing  or  de- 
creasing the  field  strength. 

The  pull*  of  the  motor  armature  is  di- 
rectly proportional  to  the  strength  of  the 
armature  current  and  to  the  strength  of 
the  magnetic  field.  In  the  case  of  the 
shunt  wound  motor  where  the  field  is  of 
constant  strength,  the  pull  of  the  arma- 
ture depends  upon  the- amount  of  current 
through  its  winding.  Hence  if  we  weaken 
the  field,  the  reduced  counter  E.  M.  F.  will 
permit  increased  flow  of  current  in  the 
armature,  and  therefore,  will  increase  its 
speed. 

The  speed  of  a  shunt  wound  motor  is 
self-adjusted  in  the  following  manner:  If  a  load  is  thrown  on  suddenly,  the  armature  will 
have  a  tendency  to  slow  down,  but  this  decreases  the  reverse  electromotive  force  and  there- 
fore increases  the  current  flowing  through  the  armature  winding.  This  causes  the  motor  to 
return  to  its  normal  speed  of  rotation. 

We  see  from  the  foregoing  that  the  speed  of  a  motor  can  be  regulated  in  two 
ways :  ( 1 )  by  connecting  a  variable  resistance  coil  known  as  a  field  rheostat  in 
series  with  the  field  winding;  (2)  by  connecting  a  variable  resistance  of  large 
current-carrying  capacity  in  series  with  the  external  circuit  or  in  series  with  the 
circuit  to  the  armature  itself. 

The  motors  of  the  motor  generators  used  in  wireless  telegraphy  are  designed 
to  permit  variation  of  the  speed  20%  above  and  below  the  normal  speed. 

40.  Motor  with  Differential  Field  Winding. — As  we  have  explained,  the 
speed  of  a  motor  is  increased  or  decreased  by  regulation  of  the  strength  of  the 
magnetic  field  and  any  reduction  of  field  flux  of  a  given  machine  will  increase 
the  speed  of  the  armature.  By  the  use  of  the  differential  field  winding  shown  in 
Fig.  29,  the  flux  of  the  shunt  field  is  automatically  weakened  in  accordance  with 
the  external  load  and  the  speed  therefore  self-regulated.  Confining  our  vision 
strictly  to  the  windings  of  the  field  poles,  two  distinct  set  of  coils  will  be  seen, 
one  a  series  winding  (SR)  in  series  with  the  armature  and  the  other  a  shunt 
winding  (SH)  connected  across  the  main  power  line.  If  the  current  in  these 
two  windings  circulates  in  opposite  directions,  a  differential  field  is  produced 
and  the  resultant  field  will  be  of  greater  or  less  intensity  according  to  the  current 

*The  term  "torque"  is  applied  to  the  twisting  force  produced  in  the  armature  when  the  current 
is  turned  on.  "Torque"  is  the  result  of  "pull"  and  "leverage." 


110  VOLT  D.C 
Fig.   29— Motor  with   Differential  Field  Winding. 


ELECTROMAGNETIC   INDUCTION. 


31 


taken  by  the  armature.  A  suddenly  applied  load  will  tend  to  slow  the  armature 
down,  and  this  will  reduce  the  counter  E.  M.  F.  of  the  armature  coils ;  accord- 
ingly increased  current  will  flow  through  the  series  winding,  which  will  reduce 
the  counter  E.  M.  F.  to  a  still  lower  figure,  permitting  such  increase  of  armature 
current  as  will  restore  the  motor  to  normal  speed. 

Through  use  of  the  differential  winding  motors  may  be  designed  to  give 
very  close  speed  regulation  and  are  therefore  distinctly  suitable  to  drive  the  A.  C. 
generators  for  wireless  telegraphy. 

If  we  keep  before  us  the  fact  that  the  counter  electromotive  force  developed 
in  a  motor  armature  acts  effectively  as  a  resistance  to  the  flow  of  current,  and 
that  this  reverse  electromotive  force  increases  with  the  speed,  it  is  easily  seen 
that  a  considerable  difference  must  exist  between  the  armature  resistance  when 
standing  still  and  its  effective  resistance  when  in  rotation.  If  such  a  motor 
armature  were  started  by  connecting  its  terminals  directly  to  the  power  mains 
an  excessive  current  would  flow  which  would  do  injury  to  the  windings  or  the 
commutator.  A  device  known  as  a  motor  starter  is,  therefore  required  to  reduce 
the  starting  current  to  a  safe  value. 

Motor  starters  will  be  treated  in  detail  in  Part  IV7,  paragraphs  55,  56  and  57. 

41.  Dynamo  and  Motor  Armatures. — Armatures  may  be  classified   with 

A  particular    reference    to    their    shapes,    the 

r \     t \         two    principal    types    being   known    as    the 

I  El_UIIIIIII!lllllllllllllllinilllllllllllllllllll^  Tr'd,?gram,aFidg.1of 

outline   of  the  drum  wound  armature,  the 

A  »• V     I  ||  I/  core  of  which  is  made  up  of  a  number  of 

Nlllllllllllllllllllllllllllllllllllllllllllllllllllllly  thin   sheets    of    soft   iron   mounted   on   the 

* '  shaft  B  to  form  the  support  for  armature 

Fig.  30-Gencral  Outline  of  Drum  Wound  Armature.        coils         The     coUs     f()r     ^     armature     arc 

placed  lengthwise  in  slots,  one  coil  being  shown  as  at  A,  B.  One  terminal,  A,  B,  is  con- 
nected to  a  segment  of  the  commutator  and  the  winding  continues  through  a  slot  to  the 
rear  of  the  armature  core  underneath  a  south  pole,  and  returns  in  the  case  of  a  four-pole 
dynamo  or  motor  about  90°  away  or  underneath  the  north  pole  where  the  second  terminal 
is  attached  to  the  next  adjacent  commutator  segment.  A  number  of  these  coils  are  con- 
nected in  series,  taps  being  brought  from  the  terminals  of  each  coil  to  the  successive  seg- 
ments of  the  commutator.  The  iron  punchings  of  the  core  0  are  insulated  from  one  an- 
other by  shellac  or  varnish  to  prevent  the  induction  of  current  in  the  core  as  well  as  in 
the  armature  coils.  A  solid  core  would  occasion  great  energy  losses  in  this  way. 

An  armature  coil  constructed  of  thin  discs  or  punchings  is  said  to  be  laminated.  The 
field  poles  and  armatures  of  both  dynamos  and  motors  are  laminated  to  prevent  induction 
losses. 

In  the  ring  wound  armature  shown  in  Fig.  31,  the  armature  conductors  are  wound  about 
a  ring-shaped  iron  core,  separated  from  one  another  and  equally  spaced,  the  term- 
inals of  each  coil  being  connected  to  ad- 
jacent segments  of  the  commutator.  Be- 
cause the  conductors  on  the  outside 
surface  of  the  core  only  are  active  in  cut- 
ting the  lines  of  force,  the  ring  wound 
armature  is  more  or  less  wasteful  and  is 
seldom  encountered  in  wireless  telegraph 
installations. 

42.  Development    of    Armature 
Windings. — The   subject   of  arma- 
ture windings  is  too  comprehensive 
to  be  treated  in  detail  here.    These 
windings  are  exhaustively  covered 
in  many  textbooks  on  dynamo  engi- 
neering which  should  be  referred  to 

for  additional   details.     The  drum       Fi    31_Gen,rai  Outlin?  of  Ring  Woun<J  Armatu 


32 


PRACTICAL  WIRELESS  TELEGRAPHY. 


armature  windings  may  be  classed  into  two  principal  types,  the  lap  winding  and 
the  wave  winding. 

The  development  of  the  lap  winding  is  shown  in  Fig.  32,  where  a  number  of  armature 
coils,  numbered  1,  2,  3,  4,  5,  etc.,  successively,  are  assumed  to  be  mounted  on  the  armature 
of  a  dynamo  and  to  cut  through  the  magnetic  fields  of  the  poles  N.  S,  N.  >S.  The  arrow 
indicates  the  direction  of  rotation,  and  the  letters  correspond  to  the  segments  of  the  com- 
mutator. The  position  of  the  positive  (-f )  brush  of  the  armature  is  shown. 

In  this  diagram,  an  armature  having  18  conductors  revolves  in  a  four  pole  field  and  the 
flow  of  current  will  be  observed  to  have  the  following  direction.  If  we  start  from  com- 
mutator segment  I,  the  point  where  the  current  enters  the  armature  through  the  negative 
brush,  then  the  current  flows  through  conductor  17,  through  commutator  segment  A, 
through  conductors  I  and  6,  and  out  at  segment  B.  The  current  is  thus  seen  to  take  two 
paths  from  the  negative  brush  through  the  armature  coils  to  the  positive  brush.  And  it 
will  be  clear  also  that  one  side  of  a  given  armature  coil  lies  underneath  a  north  pole  and 
the  opposite  side  underneath  a  south  pole  90°  distant.  The  student  should  note  carefully 
the  direction  of  the  flow  of  current  in  all  coils  of  the  armature  winding,  taking  particular 
note  of  the  fact  that  in  parts  of  the  armature,  the  current  is  flowing  towards  the  positive 
brushes  and  in  other  parts,  away  from  the  negative  brushes.  The  coils  composing  the 


Fig.  32— Development  of  Lap  Winding  on  Armature. 

armature  winding  are  connected  so  that  the  voltage  induced  in  one  adds  on  to  that  of  the 
next  coil,  hence,  current  flows  in  the  same  general  direction  through  various  groups  of 
coils  although  the  sides  of  a  given  coil  are  under  magnetic  fields  of  opposite  polarity. 

It  is  to  be  noted  that  the  brush  shown  in  Fig.  32  short  circuits  a  particular 
coil  of  the  armature,  which  lies  in  the  neutral  magnetic  field.  It  is  self-evident 
that  if  a  given  armature  coil  were  short  circuited  by  a  brush  when  the  coil  occu- 
pies a  position  other  than  the  neutral  position,  it  would  be  surrounded  by  a  mag- 
netic field  and  current  of  great  strength  -would  be  induced  therein.  This  would 
overheat  or  melt  the  conductors  or  at  least  would  cause  destructive  sparking  at 
the  commutator. 

If  the  armature  wiring  in  Fig.  32  is  carefully  traced  out,  it  will  be  observed 
that  the  winding,  so  to  speak,  laps  back  upon  itself.  It  is  therefore  termed  the 
lap  winding.  In  a  four-pole  generator,  four  brushes  would  be  required  for  this 
winding  and  in  a  six-pole  generator,  six  brushes. 

In  the  diagram  of  Fig.  32  the  coils  of  the  armature  are  shown  as  consisting 
of  a  single  turn  of  wire  but  they  may  have  several  turns  between  segments  as 


ELECTROMAGNETIC   INDUCTION. 


33 


shown  in   Fig.   33   where  a  single  coil   is   connected   to   commutator   segments 

B  and  C. 

Fig.  34  shows  the  development  of  the  so-called  wave  winding.     The  path  of  the  current 

is  as  follows :       Current  enters   commutator  segment   I,   continuing  through   conductor   17, 

or  to  segment  E,  continuing  through  conductors  9,  14,  finally 
coming  out  at  segment  A.  The  current  having  passed 
through  a  conductor  under  each  field  pole,  it  returns  to  the 
commutator  segment  A,  the  one  adjacent  to  segment  I,  at 
which  it  originally  started.  There  are  but  two  paths  for  the 
current  through  the  armature,  hence  but  two  contact  brushes 
are  required.  The  majority  of  motors  encountered  in  wire- 
less work  have  lap  wound  armatures. 

The  general  construction  of  a  drum-wound  armature  is 
shown  in  Figs.  35  and  36.  Fig.  35  shows  a  complete  Crocker- 
Wheeler  motor  generator  armature.  It  should  be  observed 
that  the  D.  C.  armature  coils  lie  lengthwise  in  slots  on  the 
iron  core  and  their  terminals  are  soldered  in  slots  at  the  end 
of  each  commutator  segment.  Fig.  36  shows  the  terminals 
of  the  armature  coils  placed  in  the  slots  ready  for  soldering. 
The  construction  of  the  commutator  should  be  noted.  It  is 

_     made    up    of    a    number    of    copper    bars    separated    by    fiber 

J     insulating  material. 


I     B      I     C      I 

COMMUTATOR 


43.  The  Alternating  Current  Transformer.— We 

011     have  shown  in  paragraphs  28  and  29  how  a  varying 

magnetic  field  threading  in  and  out  of  a  coil  of  wire  wound  over  an  iron  core  can 
induce  a  flow  of  current  into  another  coil  wound  about  it.  Mention  was  made  of 
the  fact  that  direct  current  flowing  through  the  first  coil  must  be  interrupted  or 
its  strength  changed  periodically  to  induce  a  current  in  the  second  coil. 

It  is  clear  from  Fig.  19,  that  the  lines  of  force  produced  by  winding  P  cut  each 
turn  in  S  just  once,  and,  therefore,  the  pressure  or  electromotive  force  induced 


Fig.  34 — Development  of  Wave  Winding. 

in  winding  S  increases  or  decreases  accordingly  as  the  number  of  turns  in  S  are 
greater  or  less  than  in  P  (see  paragraph  31). 

The  apparatus  built  upon  this  principle  is  known  as  a  transformer  and  the 
different  types  are  called  step-up  or  step-down  with  respect  to  the  ratio  of  the 
primary  and  secondary  turns. 


34 


PRACTICAL  WIRELESS  TELEGRAPHY. 


The  essentials  of  a  transformer  are: 

(1)  A  primary  winding; 

(2)  A  secondary  winding; 

(3)  An  iron  core. 

In  order  that  the  current  may  be  induced  in  the  secondary  of  a  transformer,  the 
primary  winding  must  be  traversed  by  either  a  pulsating  or  interrupted  direct  current 
or  an  alternating  current. 

Fig.    35 — Construction    of    Crocker    Wheeler    Motor    Generator    Armature. 


A.  CARICATURE 


W.L!CTOR 
RINGS 


Fig.  36 — Showing  Crocker  Wheeler  Motor  Armature   with  Coil  Terminals  Unsoldered. 

Alternating  current  transformers  for  the  production  of  high  voltages  may  be  broadly 
classified  under  two  general  types : 

(1)  The  constant  current  transformer; 

(2)  The  constant  voltage  transformer. 
In  terms  of  the  ratio  of  transformation,  they  may  be  classified  as : 

(1)  Step-up  transformer; 

(2)  Step-down  transformer. 

According  to  the  design  of  the  coils  and  the  core  or  magnetic  circuit,  they  may  be 
classified  as : 

(1)  Open  core  transformer; 

(2)  Closed  core  transformer; 

(3)  Auto  transformer; 

(4)  Air  core  transformer. 

Fig.  37  is  an  elementary  diagram  of  a  closed  core,  step-up,  constant  voltage  transformer. 
The  primary  and  secondary  windings  P  and  S  respectively,  are  supported  by  a  rectangular 
iron  core  built  up  of  strips  of  sheet  iron.  The  primary  winding,  for  example,  may  consist 
of  one  or  two  layers  of  comparatively  coarse  wire  such  as  No.  10  or  No.  12  B.  &  S.  gauge. 
The  secondary  winding  S  may  have  several  thousand  turns  of  fine  wire  such  as  No.  30  or 
No.  32. 

The  process  of  transformation  is  as  follows :  The  alternating  current  flowing  from  a 
dynamo  through  the  primary  winding  P  magnetizes  the  iron  core  periodically,  causing  a 


ELECTROMAGNETIC   INDUCTION. 


35 


* 

1 

OLT5             C 
YCUS      P      £ 

C 

-_ 
—  —  • 

«  — 
T~ 
—  - 

) 

;i 

•  i 

.•: 

:T 

- 

• 
• 

I 

"- 

radio    work. 


varying  flux  to  flow  through  the  iron  core  in  accordance  with  the  alternations  of  current. 
This  varying  flux  induces  an  E.  M.  F.  in  the  secondary  which  will  cause  a  current  to  flow  if 
the  secondary  circuit  is  closed.  The  current  in  the  secondary  circuit  flows  in  the  opposite 
direction  from  that  in  the  primary  circuit  and  as  it  increases,  it  sets  up  a  flux  in  opposition 
to  that  already  in  the  core,  reducing  its  strength.  This  reduces  the  self-induction  of  the 
primary,  permitting  more  current  to  flow 
in  the  primary  and  in  this  way  the 
transformer  becomes  self-regulating — 
a  rise  of  the  secondary  current  causing 
an  increase  in  the  primary  current. 

If,  for  example,  current  at  110 
volts,  500  cycles,  flows  through 
winding  P,  the  flux  will  alternate 
through  the  core  1,000  times  per 

SeCOnd,     Setting     Up      1,000     alteriia-  Fig.    37-Mnguetie    Circuit   of   Step-up   Closed   Core 

tions    of    current    in    winding    S.  Transformer. 

Since  S  consists  of  a  great  number  of  turns,  the  voltage  of  the  current  induced  in 
S  will  be  very  much  greater  than  the  voltage  of  the  current  in  winding  P.     In 
fact,  it  is  found  that  the  voltage  in  the  secondary  winding  is  almost  a  direct 
ratio  of  the  primary  and  secondary  turns,  e.  g., 
E-s       T-s 

E-p  ~T-p 

where  T-p  =  the  current  in  the  primary ; 
where  T-s  =  the  current  in  the  secondary ; 

E-p  and  E-s  =  the  voltage  in  the  primary  and  secondary  circuits  respectively. 
Hg.   38   shows   the   open   core,   step-up    voltage,   constant   current   transformer  employed 
C,    the    core    constructed    of    a    bundle    of    fine    iron    wires    or    of    sheet 

iron  is  covered  with  several  layers  of 
insulating  cloth  followed  by  one  or  two 
layers  of  coarse  copper  wire.  An  insu- 
lating tube  (not  shown)  is  placed  over 
the  primary.  It  is  made  of  some  mate- 
rial which  will  withstand  the  heat  and 
possess  the  requisite  insulating  qualities. 
Over  this  tube  is  placed  a  secondary 
winding  which  consists  of  several 
thousand  turns  of  fine  wire  wound  up  in 
the  form  of  pancakes  as  at  S-l,  S-2,  S-3. 
There  is  little  magnetic  reaction  of  the 
secondary  upon  the  primary  in  this  type 
of  transformer  owing  to  the  lack  of  a 
continuous  iron  path  for  the  flux  and  the 
self-induction  of  the  primary  therefore 
remains  nearly  constant.  The  transform- 
er will  draw  practically  the  same  current 
when  the  secondary  is  on  short  circuit  as  when  it  is  open. 

The  closed  core  transformer  can  be  designed  to  have  this  operating  characteristic  when 
fitted  with  a  magnetic  leakage  gap  shown  by  the  dotted  line,  Fig.  37.  The  reactive  lines 
of  force  from  the  secondary  pass  through  this  gap  and  the  self-inductance  of  the  primary 
winding,  therefore,  remains  nearly  constant  under  all  variations  of  the  secondary  load. 

Both  the  open  and  closed  core  transformers  are  employed  in  wireless  telegraphy  to 
generate  current  at  voltages  between  teii  thousand  and  fifty  thousand  volts  at  power  input 
varying  from  Y4  K.  W.  to  500  K.  W. 

The  ratio  of  transformation  in  the  open  core  transformer  is  not  exactly  in  proportion 
to  the  turns,  due  to  magnetic  leakage.  The  design  is,  therefore,  altered  to  meet  these 
conditions.  Generally  the  secondary  is  given  more  turns  than  the  usual  transformer  equation 
would  require. 

The  so-called  auto  transformer  with  a  step-up  ratio  of  turns  is  shown  in  Fig.  39.  In 
this  type  the  primary  and  secondary  windings  have  turns  in  common,  a  single  coil  being 


Fig.    38 — Open    Core    Transformer. 


36 


PRACTICAL  WIRELESS  TELEGRAPHY. 


39 — Auto  Transformer, 
copper  strips,  the  tubing  being  from  l/>  to 


used  for  both  circuits.  A  portion  of  the  current  flowing  in  the  secondary  winding  is  in- 
duced by  the  passing  of  the  flux  through  the  core  from  the  primary  turns,  but  another 
portion  flows  into  the  secondary  circuit  by  direction  conduction. 

Although  transformers  of  this  type  are  not  employed  for  the  production  of  high  voltages 
(with    low    frequency   currents),    they    are    frequently    used    as    step-down    transformers    to 

obtain  10  to  30  volts  of  alternating  cur- 
rent from  a  110- volt  source.  Without  an 
iron  core,  auto  transformers  are  used  in 
the  circuits  of  radio  frequency  in  both 
the  transmitting  and  receiving  apparatus 
of  wireless  telegraphy. 

The  air  core  transformer  in  Fig.  40  is 
used  principally  in  radio-frequency  cir- 
cuits for  transferring  oscillations  at  ex- 
tremely high  frequencies  from  one  circuit 
to  another,  and  when  used  in  this  man- 
ner it  might  properly  be  called  a  radio- 
frequency  transformer.  For  such  a  trans- 
former if  used  in  the  transmitting  ap- 
paratus of  a  radio  set,  winding  P  is  made 
of  a  few  turns  of  coarse  copper  tubing  or 
inch  in  diameter  for  the  small  size  sets.  Wind- 
ing S  may  have  several  turns,  a  dozen  or  more  of  insulated  wire  or  small  copper  tubing. 
On  the  other  hand,  if  the  auto  transformer  is  used  in  receiving  sets,  winding  P  may  consist 
of  several  hundred  turns  of  No.  24  B.  &  S.  wire,  and  winding  S  may  have  several  hundred 
turns  of  No.  32  B.  &  S.  wire. 

Whether  or  not  the  voltage  of  the  sec- 
ondary circuit  will  be  greater  or  less  than  in 
the  primary,  in  radio-frequency  transforma- 
tion, depends  upon  the  values  of  capacity 
included  in  either  circuit  as  well  as  the  ratio 
of  the  turns.  Owing  to  the  phenomenon  of 
resonance  and  the  effects  of  capacity,  a  step- 
up  ratio  of  turns  may  be  the  equivalent  of  a 
step-down  voltage  or  vice  versa. 

When  the  primary  circuit  of  an  open  core 
transformer  is  supplied  with  interrupted  di- 
rect current,  it  is  called  an  induction  coil. 
This  coil  will  be  described  in  detail  in  Para- 
graph 50. 

44.  Electrostatic  Capacity.— In  or- 
der that  certain  phenomena  involved  in 
the  flow  of  alternating  current  may  be 
understood,  it  will  be  necessary  to  con- 
sider another  quality  of  an  electric  cir- 


30,000  TO 
1,000.000 
CYCLES 


SECONDARY 


PRIMARY 


Fig.   40 — Radio   Frequency   Transformer. 


cuit  known  as  electrostatic  capacity.  We  have  mentioned  two  qualities  of  an 
electric  circuit,  i.  e.,  resistance  and  inductance.  The  third  quality,  capacity,  is  of 
particular  importance  in  wire  telegraph  apparatus  and  will  now  be  defined. 

Further    on,    we    shall    show    how 

c  these  three  qualities  govern  the  flow 

of  an  alternating  current. 

Capacity  may  be  defined  as  that 
property  of  a  conductor  or  circuit 
by  which  energy  can  be  stored  up  in 
electrostatic  form.  The  electrostatic 
capacity  of  a  conductor  is  measured 
by  the  quantity  of  electricity  in 
coulombs  with  which  it  must  be 
charged  to  raise  its  potential  to  one 
volt/ 


EFFECTIVE    VALUE     10  5  AMP 


Fig.  41— Rise  and  Fall  of  Alternating  Current. 


ELECTROMAGNETIC   INDUCTION. 


STATIC  FIELD 


Fig.   42 — Simple   Condenser. 


A  device  for  storing  up  energy  in  the  form  of  an  ^electrostatic  field  is  known  as  a 
condenser.  When  two  copper  sheets  or  other  conducting  material  are  separated  by  a 
small  air  space  as  in  Fig.  42,  and  a  source  of  direct  or  alternating  current  connected  to 

the  two  plates,  the  intervening  space  fills 
up  with  electrostatic  lines  of  force.  If 
the  charging  source  be  disconnected,  and 
the  terminals  of  the  condenser  be  recon- 
nected to  a  galvanometer,  the  latter  will 
give  a  momentary  deflection  indicating 
the  passage  of  an  electric  current.  This 
experiment  proves  that  the  electrostatic 
field  within  a  condenser  will,  when  re- 
leased, set  up  a  flow  of  an  electric  current. 
The  unit  for  expressing  the  ca- 
pacity of  a  condenser  is  the  farad 
which  is  a  condenser  of  such  dimen- 
sions that  one  volt  of  electricity  will 
store  up  in  it  a  charge  of  one 
coulomb.  The  farad  is  too  large  for  practical  measurements,  hence  the  micro- 

1 
farad  is  in  general  use.     One  (1)  microfarad  =  -  farad. 

1,000,000 

The  quantity  of  electricity  that  can  be  placed  in  a  condenser  is  directly  proportional  to 
its  capacity  and  the  difference  of  potential  between  its  plates,  or  Q  —  C  X  E-  Hence,  a 
condenser  of  .000002  farad  capacity,  charged  to  a  potential  of  10,000  volts  would  have 
stored  up  in  it  10,000  X  .000002  —  .02  coulomb. 

When  current  flows  into  a  condenser,  its  potential  difference  rises  uniformly  until  the 
E.  M.  E.  of  the  condenser  and  that  of  the  charging  source  are  equal.  At  any  instant, 

Q 

the  E.  M.  F.  of  the  condenser  is  proportional  to  —  but  since  the  charging  process  is  uniform 

C 
E 

the  average  E.  M.  F.  =  — . 
2 

The  work  done  in  joules  in  placing  a  quantity  of  electricity  into  a  condenser  —  the 
quantity  of  electricity  multiplied  by  the  average  E.  M.  F.,  hence 

E 

the  work  in  joules  =  Q  X  — 

2 
Since  Q  =  E  X  C 


C  E2 


therefore  W  —  EXCX  —  — 


2  2 

Where  W  =  the  work  in  joules. 
Now  if  a  condenser  is  charged  N  times  per  second,  the  power  is  expressed: 

C  E2 
P=i-      -  X  N 

2 
.    Where  N  =  the  number  of  charges  per  second  ; 

C  =i  the  capacity  of  the  condenser  in  farads ; 
E  —  the  potential  difference  in  volts. 

Hence  if  a  condenser  of  .002  microfarad  capacity  were  charged  to  a  potential  of  30,000 
volts  by  a  500  cycle  alternator,  the  power  expended  in  watts   would  be : 
.000000002  X  30,0002 

•    X  1,000  =  900  watts. 
2 
A  condenser  of  concentrated   capacity  always  consists   of : 

(1)  Two  or  more  opposing  surfaces; 

(2)  An  insulating  medium  between  the  plates  which  may  be  air  or  any  of  the  well- 

known  insulating  materials,  such  as  glass,  micanite,  hard  rubber,  waxed  paper, 
etc. 


38  PRACTICAL  WIRELESS  TELEGRAPHY. 

This  medium  is  known  as  the  dielectric.     The  capacity  of  a  condenser  is  found  to  vary: 

(1)  Directly  as  the  area  of  the  opposed  surface  and  the  ability  of  the   dielectric  to 

conduct  electrostatic  lines  pf  force; 

(2)  Inversely  as  the  separation  of  the  plates. 

This  may  he  written  : 

K  X  A  X  2248 


T  X  1010 
Where  C  =  the  capacity  of  the  condenser  in  microfarads  ; 

A  =  the  area  of  the  opposed  surfaces  in  squares   inches  ; 

K  =  a  certain  constant  ; 

T  =  the  separation  of  the  opposed  surfaces,  or  the  thickness  of  the  di-electric. 

It  can  be  proven  that  different  di-electric  mediums  conduct  static  lines  of  force 
with  more  or  less  ease  depending  upon  their  nature.  Air  is  taken  as  unity  and 
all  other  insulating  mediums  are  compared  to  it.  Certain  grades  of  glass  are 
said  to  have  a  dielectric  constant  of  9,  meaning  that  a  condenser  with  a  plate  of 
glass  between  conducting  surfaces  will  permit  9  times  the  quantity  of  electricity 
to  be  stored  up  as  with  air  at  ordinary  pressure.  In  the  same  way,  the  dielectric 
constant  of  micanite  is  said  to  be  5,  paraffin  paper  2,  etc.  (Note  complete  table  in 
the  Appendix). 

Condensers  of  large  capacity  are  made  by  taking  a  number  of  sheets  of  tin  or  brass 
foil  and  separating  them  with  thin  sheets  of  waxed  paper  or  other  insulating  material, 
alternate  sheets  of  foil  being  connected  together  on  either  side,  so  there  is  no  direct  con- 
nection between  them.  This  constitutes  a  condenser  of  concentrated  capacity  which  may 
store  up  temporarily  considerable  amounts  of  energy  in  electrostatic  form. 

Condensers  may  be  classified  with  respect  to  their  dielectric  strength  which  may  be 
defined  as  the  ability  of  the  dielectric  to  resist  puncture  when  subjected  to  electric  pressure. 
Condensers  which  will  withstand  high  voltages  without  rupture  of  the  dielectric  are  termed 
high  potential  condensers  and  conversely  those  which  will  withstand  low  voltages  only  are 
called  low  potential  condensers.  High  voltage  condensers  are  used  in  circuits  of  several 
thousand  volts  pressure.  Low  voltage  condensers  are  employed  in  circuits  of  less  than 
500  volt  pressure. 

We  have  mentioned  that  a  condenser  when  first  connected  to  a  charging 
source,  has  zero  potential,  and  as  the  current  flows,  the  potential  difference  rises 
until  the  voltage  of  the  condenser  is  equal  to  voltage  of  the  charging  circuit  ;  the 
flow  of  current  then  stops.  If  the  applied  potential  is  decreased,  the  condenser 
will  start  to  discharge  and  current  will  flow  out  in  the  opposite  direction  to 
which  it  was  charged.  The  voltage  of  the  condenser  is  thus  seen  to  set  up  a 
back  pressure  which  tends  to  drive  the  charging  current  back. 

We  have  already  seen  how  inductance  tends  to  prolong  the  flow  of  current  in 
a  circuit  and  we  now  see  that  the  condenser  tends  to  extinguish  it  or  drive  it 
back.  Thus  the  back  pressure  of  the  condenser  opposes  that  set  up  by  an  in- 
ductance coil.  We  shall  now  see  how  these  counter  E.  M.  F.'s  govern  the  flow  of 
alternating  current. 

The  effects  of  self-inductance  will  first  be  noted. 

45.  Reactance  and  Impedance.  —  When  a  coil  of  wire  is  connected  to  a 
source  of  direct  and  then  to  a  source  of  alternating  current  of  the  same  voltage, 
the  flow  of  current  (in  amperes)  will  be  considerably  greater  with  the  former 
connection  than  with  the  latter.  This  is  due  to  the  fact  that  the  counter  E.  M.  F. 
of  self-induction  in  a  direct  current  circuit  is  only  momentary,  the  effects  being 
observed  when  the  current  is  turned  on  and  off,  whereas  in  a  circuit  carrying 
alternating  current,  the  effects  of  self-induction  are  continuous  and  the  back 
pressure  resulting  therefrom  must  always  be  considered  to  determine  the  strength 
of  current. 

The  flow  of  a  direct  current  through  a  given  circuit  is  opposed  only  by  the 
ohmic  resistance,  but  the  flow  of  alternating  current  is  impeded  by  the  counter 
electromotive  force  of  self-induction  as  well  as  by  the  ordinary  resistance.  The 


ELECTROMAGNETIC   INDUCTION. 


39 


extra  resistance  of  self-induction  is  termed  reactance,  and  is  expressed  in  equiva- 
lent ohms.  The  combined  opposition  of  reactance  and  resistance  in  any  circuit  is 
termed  impedance,  and  accordingly  the  flow  of  current  through  a  circuit  carrying 
alternating  current  is  governed  by  the  impedance  and  not  alone  by  the  ohmic 
resistance.  It  should  be  understood  that  the  counter  E.  M.  F.  of  self-induction 
entails  no  loss  of  energy  in  an  electric  circuit  as  does  resistance  (where  the 
energy  is  lost  in  the  form  of  heat),  but  a  higher  voltage  is  required  in  that 
circuit  to  force  a  given  value  of  current  through  it. 

The  flow  of  alternating  current  is  nearly  always  controlled  by  coils  of  high 
self-induction  which  are  termed  reactance  coils  or  "choking"  coils. 

The    reactive    pressure    occasioned    by   a    circuit    loaded    with    inductance   is    termed    in- 
ductance reactance.     It  is  expressed  : 

Reactance  =  6.28  X  N  X  L 
Where  N  —  the  frequency  in  cycles  per  second. 
L  =.the  inductance  in  henries. 

If  the  coil  L  of  Fig.  43  has  inductance 
of  .055  henry,  and  it  is  connected  to  a  60- 
cyqje  alternator,  the  inductive  reactance 
=  6.28  X  60  X  -055  =  20  ohms.  If  the 
frequency  be  increased,  the  reactance  (in 
ohms)  increases  in  the  direct  ratio;  thus 
if  N  ==  100,000  cycles,  the  frequency  of 
a  radio-frequency  alternator,  then  the 
reactance  of  coil  L  =  34,540  ohms. 

The  flow  of  current  through  L  is 
governed  both  by  the  reactance  and  the 
resistance,  and  the  impedance  of  such  a 
circuit  is  expressed  as  follows  : 


60  CYCLES 
A  C 


43 — -Alternating    Current    Circuit    with    Concentrated 
Inductance. 


Impedance  =  V 

Where  R  =  the  resistance  of  the  coil  in  ohms; 

X  =  the  reactance  of  the  coil  in  ohms. 

Then  if  the  coil  L  of  Fig.  43  had  inductance  of  .055  henry,  resistance  of  10  ohms  and 
reactance  of  20  ohms,  then 

Impedance  =  V  202Xl02  =  22.3  ohms  approximately. 

E 
For  direct  or  continuous  current,  Ohm's  law  is  expressed  I  —  —  ,   but  for  alternating 

R 
E 
current  the  formula  is  modified  to,  I  =r  —  .     If  the  pressure  of  the  alternator  is  110  volts, 

Z 

110 
then  there  will  flow  through  L,  -     -   —  4.9  amperes  nearly. 

22.3 

46.  Capacity  Reactance.  —  We  have  shown  how  a  condenser  connected  in 
series  with  an  alternating  current  circuit  acts  as  an  effective  resistance  and  exerts 
a  back  pressure  on  the  charging  E.  M.  F.,  and  also  that  this  back  pressure  opposes 
that  set  up  by  inductance.  To  distinguish  these  counter  E.  M.'F/s,  the  reactance 
occasioned  by  inductance  is  expressed  as  positive  reactance  and  that  by  a  con- 
denser as  negative  reactance, 

The  capacity  reactance  of  a  condenser  is  determined  as  follows  : 

1 
Capacity  reactance  =  — 

6.28  X  N  X  C 
Where  N  =  the  frequency  of  the  current  in  cycles  per  second  ; 

C  =  the  capacity  of  the  condenser  in  farads. 

The  important  point  to  be  noted  from  this  formula  is  that  a  large  condenser  will  have 
a  small  value  of  reactance  and  conversely  a  small  condenser  will  have  a  large  value  of 
reactance. 


40 


PRACTICAL  WIRELESS  TELEGRAPHY. 


If  the  condenser  C  connected  in  series  with  the  60  cycle  alternator  of  Fig.  44  has  capacity 
of  .00013  farads,  and  the  frequency  of  the  alternator  is  60  cycles,  then 

1 

Capacity  Reactance  = =  20  ohms  approximately. 

6.28  X  60  X  .00013 

If  the  frequency  of  the  alternator  is  100,000  cycles,  then, 

1 


Capacity  Reactance  = 


—  =  .012  ohms. 


6.28  X  100,000  X  -00013 

It  is  clear  that  by  proper  selection  of  capacity  and  inductance  values  in  the  alternator  circuit 
of  Fig.  45,  the  counter  electromotive  forces  can  be  made  to  balance  and  the  reactance  there- 

1 
fore  reduces  to  zero,  or  2ir  N,  L  =  — 

27T  N,C 

The  circuit  then  acts  as  if  neither  induct- 
ance or  capacity  were  present  and  the 
flow  of  current  is  governed  solely  by  the 
ohmic  resistance  of  the  circuit. 

If  capacity  reactance  overbalances  in- 
ductance reactance,  then  the  resultant 
value  takes  the  notation  of  the  predom- 
inating figure,  e.  g.,  if  the  capacity  react- 


60  CYCLES 
A  C 


Fig.  44 — Alternating  Current  Circuit  with  Condenser 
Series. 


>"  ance  exceeds  the  inductance  reactance,  the 
difference  between  the  two  will  be  ex- 
pressed in  ohms,  and  the  circuit  said  to  have  so  many  ohms  capacity  reactance.  In  case 
inductance  reactance  predominates,  the  opposite  statement  applies.  We  see  from  all  this 
that  a  much  greater  current  can  be  made  to  flow  through  the  circuit  from  the  alternator 
by  the  use  of  a  condenser  and  a  coil  than  if  but  one  of  these  were  used. 

Reviewing  the  foregoing,  it  is  clear  that  the  reactance  of  a  given  coil  for  frequencies 
in  excess  of  100,000  cycles  per  second  (as  compared  to  lower  frequencies)  may  attain  a 
rather  large  value.  It  is  therefore  nec- 
essary in  such  circuits  to  insert  a  cer- 
tain amount  of  concentrated  capacity  to 
build  up  the  current.  In  the  radio- 
frequency  circuits  of  wireless  telegraph 
apparatus,  current  flows  at  frequencies 
between  20,000  and  1,000,000  cycles  per 
second  and  if  this  current  is  to  be  trans- 
ferred by  magnetic  induction  from  one 
circuit  to  another,  the  second  circuit 
must  contain  a  certain  amount  of  in- 
ductance and  capacity  of  such  values 
that  inductance  reactance  and  capacity 
reactance  neutralize  one  another.  The 


60  CYCLES 
A.C 


Fig.   45 — Showing  How  Resonance  Is   Obtained  in   Alter- 
nating Current  Circuits. 


second  circuit  is  then  said  to  be  resonant  to  the  impressed  frequency  and  the  flow  of  current  is 
governed  solely  by  its  resistance. 

Straight  wires  possess  both  capacity  and  inductance,  which  are  said  to  be  dis- 
tributed rather  than  concentrated  as  in  the  case  of  a  condenser  or  a  coil  of  wire 
The  laws  of  electrical  resonance  apply  to  such  circuits  as  well  as  those  having 
concentrated  capacity  and  inductance. 

47.  Lag  and  Lead  of  Alternating  Current. — A  certain  phenomenon,  in- 
volved in  the  flow  of  alternating  current  throughout  a  given  circuit,  is  termed 
phase  displacement.  Given  a  circuit  in  which  inductance  reactance  predominates, 
it  is  found  that  when  a  given  alternating  electromotive  force  is  applied  thereto,  the 
pressure  and  current  do  not  reach  their  maximum  values  simultaneously.  The 
current  lags  behind  the  impressed  voltage  by  a  certain  degree  dependent  upon  the 
self-induction  of  the  circuit  and  such  a  circuit  is  said  to  haye  a  lagging  phase. 

As  it  is  convenient  to  express  a  complete  cycle  of  current  in  terms  of  the  degrees  of  a 
circle,  l/4  cycle  being  equivalent  to  90°,  l/i  cycle  to  180°,  and  so  on,  we  express  the  lag  of 
the  current  in  terms  of  the  degrees  of  the  circle.  Hence,  a  certain  circuit  is  said  to  have 


ELECTROMAGNETIC   INDUCTION.  41 

an  angle  of  lag  of  35°  or  some  other  degree  dependent  upon  the  constants  of  the  circuit. 
(All  this  is  explained  in  any  strictly  theoretical  text  book  on  alternating  current). 

In  a  circuit  wherein  capacity  reactance  predominates,  the  opposite  condition  is  obtained, 
e.  g.,  the  current  leads  the  voltage  reaching  its  maximuhi  value  before  the  impressed 
E.  M.  F.  A  circuit  of  this  type  is  said  to  have  a  leading  phase. 

The  point  to  be  brought  out  here  is  that  in  circuits  having  either  lead  or  lag,  the  actual 
power  consumption  in  watts  cannot  be  determined  from  the  reading  of  a  voltmeter  or 
ammeter.  To  illustrate :  When  a  voltmeter  or  ammeter  are  connected  in  the  primary  wind- 
ing of  a  high  voltage  transformer  of  the  type  used  in  wireless  telegraph  transmitters,  the 
voltmeter  may  indicate  110  volts  and  the  ammeter  current  strength  of  14  amperes.  Apply- 
ing the  power  formula  in  simple  form  the  apparent  reading  in  watts  would  be  110  X  14  = 
1,540  watts,  but  a  wattmeter  connected  in  this  circuit  may  indicate  a  reading  of  1,000  watts 
which  is  the  true  reading  because  the  wattmeter  is  constructed  to  read  correctly  inde- 
pendently of  the  degree  of  phase  displacement.  The  result  obtained  by  multiplying  the 
pressure  by  the  current  is  only  an  apparent  reading  of  watts ;  but  the  true  reading  is 
always  obtained  by  the  meter.  The  ratio  of  the  true  watts  to  the  apparent  watts  is  ex- 
pressed by  the  term,  the  power  factor.  Tn  the  circuit  taken  as  an  example,  the  power 

1000 

factor  =        — ,  or  approximately  65%. 
1540 

R 
The  power  factor  can  also  be  obtained  from  the  ratio  of  — ,  that  is  if  the  impedance 

Z 

and  resistance  are  known,  and  the  value  of  the  former  is  divided  by  the  latter,  the  power 
factor  of  the  circuit  is  obtained. 

lie  formula  for  power  in  direct  current  circuits,  W  =  I  X  E  is  changed  in  the  case 
of  an  alternating  current  circuit  wherein  the  current  lag's  behind  the  voltage  to  read : 
W  i=  I  X  E  X  Cos.  0. 

The  cosine  <i>  is  the  power  factor  expressed  as  a  function  of  an  angle  of  a  circle  and 

R 
as  mentioned,  is  equal  to  — .     Hence  if  the  total  resistance  of  a  given  circuit  is  known,  also 

Z 

the  total  impedance  and  the  reading  of  current  and  E.  M.  F.  is  obtained  by  an  ammeter  and 
voltmeter  respectively,  we  can  determine  the  true  power  in  watts  in  any  circuit  without  the 
use  of  a  wattmeter. 

48.  Effective  Value  of  Alternating  E.  M.  F.  and  Current.— It  is  self- 
evident  from  the  alternating  current  curve  of  Fig.  41  that  the  current  constantly  changes 
in  value'  as  well  as  reversing  its  direction.  Hence,  to  express  the  effectiveness  of  a  given 
electromotive  force  in  such  circuits,  we  must  employ  some  value  other  than  the  maximum 
E.  M.  F.  or  maximum  current  per  alternation.  Take,  for  example,  any  given  circuit  in 
which  the  maximum  current  for  each  alternation  amounts  to  15  amperes  (as  in  Fig.  41), 
it  is  evident  that  at  all  points  off  maximum  during  the  complete  cycle,  the  strength  of  the 
current  is  less  than  15  amperes.  It  is  clear  that  we  must  take  some  sort  of  an  average 
value  in  order  to  determine  the  effectiveness  of  an  alternating  current.  Since  the  heating 
effects  of  direct  current  in  a  given  circuit  are  uniform,  the  effectiveness  of  an  alternating  cur- 
rent is  expressed  in  terms  of  the  strength  of  a  given  amount  of  direct  current  which  would 
produce  the  same  power  or  heating  effect.  To  illustrate:  If  15  amperes  of  direct  current 
pass  through  a  resistance  of  2  ohms,  the  power  of  the  current  converted  to  heat  will  be 
\-  X  R  =  152  X  2  =  450  watts.  Now  if  we  pass  an  alternating  current  through  the 
same  wire  and  adjust  its  strength  until  450  watts  are  consumed  in  the  form  of  heat, 
we  would  then  have  15  amperes  of  alternating  current  flowing. 

This  is  the  so-called  effective  value  of  the  alternating  current  which  in  the  case  of 
a  sine  wave  curve  is  found  to  be  .707  of  the  maximum  value  per  alternation.  Suppose, 
the  maximum  value  of  current  per  alternation  in  the  curve  of  Fig.  41  is  15  amperes, 
then  the  effective  value  will  be  15  X  -707  •=.  10.5  amperes.  That  is,  the  current  rises 
and  falls  uniformly  between  a  value  of  -(-15  amperes  and  — 15  amperes  producing  the 
same  heating  effect  as  a  direct  current  of  10.5  amperes.  Now  an  ammeter  connected 
in  such  a  circuit  would  indicate  10.5  amperes  because  these  instruments  are  constructed 
to  indicate  the  effective  value  of  current  and  not  the  maximum  value  per  alternation. 
Similarly,  voltmeters  indicate  the  effective  voltage  in  a  given  circuit.  All  this  means 
that  the  maximum  voltage  per  cycle  of  an  alternating  current  supplied  from  power  mains  at 


42 


PRACTICAL  WIRELESS  TELEGRAPHY. 


pressure  of  500  volts,  is  somewhat  greater,  in  fact,  is  500  X  1.41  =  705  volts.  Similarly  the 
maximum  voltage  per  alternation  in  110  volt  alternating  current  circuits  is  155  volts.  (This 
is  only  true  when  the  wave  form  of  the  current  follows  the  curve  of  sines.) 

When  speaking  of  the  pressure  of  the  high  voltage  transformers  used  in  wireless 
telegraphy,  the  secondary  voltage  is  generally  given  as  the  maximum  voltage  per 
cycle  and  not  the  effective  value. 

The  student  will  see  from  the  foregoing  that  the  problems  of  alternating  current 
circuits  are  largely  different  than  those  of  direct  current  circuits  and  that  the  flow  of 
current  is  governed  by  conditions  other  than  the  ohmic  resistance.  Also  the  actual 
power  consumption  in  watts  depends  upon  whether  or  not  the  pressure  and  current  in  a 
given  circuit  are  in  exact  phase. 

49.  Measuring  Instruments  or  Electric  Meters. — The  principle  measuring 
instruments  employed  in  connection  with  a  wireless  telegraph  transmitter  are : 

(1)  The  voltmeter; 

(2)  The  ammeter; 

(3)  The  wattmeter; 

(4)  The  frequency  meter; 

(5)  The  hot  wire  ammeter. 

In  the  circuits  of  a  radio-transmitter,  these  instruments  occupy  the  positions  fol- 
lowing: The  voltmeter  is  joined  across  the  terminals  of  the  alternator;  the  ammeter 
is  connected  in  series  with  the  primary  winding  of  the  transformer;  the  wattmeter  is 
connected  in  the  circuit  from  the  alternator  to  the  transformer;  the  frequency  meter 
is  shunted  across  the  terminals  of  the  alternator;  the  hot  wire  ammeter  is  used  prin- 
cipally in  circuits  of  radio-frequency,  and  to  some  extent,  in  circuits  of  lower  frequency. 
Before  entering  into  a  description  of  these  meters,  we  shall  explain  the  workings 

of  the  current-detecting  instrument  knovi 
the  galvanometer.  This  instrument  may 
take  one  of  several  forms,  but  the  type 
shown  in  Fig.  46  is  the  least  difficult  to 
understand.  A  rectangular  coil  of  sev- 
eral turns  of  copper  wire  D  is  suspended 
between  the  poles  of  a  horse  shoe 
permanent  magnet,  P,  P.  Between  the 
poles  is  the  stationary  iron  core  C*  The 
coil  is  suspended  from  the  screw  F  and  the 
current  to  be  measured  enters  at  the  wire 
A  and  leaves  by  the  wire  B.  When 
current  is  passed  through  the  coil  B,  it 
tends  to  turn  so  as  to  include  the  greatest 
number  of  lines  of  force  but  is  resisted  by 
the  torsion  of  the  suspending  wires.  If  a 
pointer  and  suitable  scale  are  attached  to 
this  coil,  comparative  readings  of  the 
strength  of  current  may  be  made.  In- 
struments of  this  construction  are  sen- 
sitive and  will  easily  measure  a  current 
of  .000001  of  an  ampere.  It  is  an  impor- 
tant instrument  to  demonstrate  the  ele- 
mentary principles  of  electromagnetic 
induction,  and  should  be  a  part  of  all 
students'  equipment. 

Now  if  the  coil    D    had    several 


Fig.  46 — Simple  Galvanometer. 

thousand  ohms  resistance,  the  galvanometer  might  be  calibrated  in  volts  and  em- 
ployed as  a  voltmeter.  If,  on  the  other  hand,  coil  D  were  wound  with  a  few  turns 
of  relatively  coarse  wire,  it  might  be  calibrated  in  amperes  and  would,  therefore, 
be  known  as  an  ammeter.  As  an  ammeter  it  would  be  connected  in  series  with  the 
circuit  under  measurement. 

*The  Core  C  intensifies  the  field  across  the  air  gap  P  to  P. 


ELECTROMAGNETIC    INDUCTION. 


— vWW 


(a)  The  voltmeter  may  be  constructed  along  the  lines  of  galvanometer.     A 
simple  drawing  of  the  Weston  voltmeter  appears  in  Fig.  47. 

A  rectangular  coil  of  fine  wire,  A,  B, 
is  mounted  on  the  metal  bobbin  G.  It  is 
supported  by  jewelled  bearings  and  held 
in  the  zero  position  of  the  scale  by  the 
spiral  springs,  S-l  and  S-2,  through 
which  the  circuit  of  the  coil  is  com- 
pleted. When  the  pointer  is  in  zero  po- 
sition, the  coil  rests  at  a  slight  angle  to 
the  pole  pieces  of  a  permanent  magnet. 
N,  S.  When  current  is  flowing  through 
the  coil,  the  normal  field  from  N  to  S  is 
"lengthened"  out  and  in  trying  to  "shorten" 
themselves,  the  lines  of  force  actually 
"twist"  or  turn  the  coil.  When  the  tension 
of  the  spring  is  equal  to  the  pull  of  the 
magnetic  field,  the  pointer  comes  to  rest 
and  the  reading  of  the  instrument  may  then 
he  observed.  An  external  resistance  coil, 
R,  is  connected  in  series  with  the  winding 
of  the  bobbin  to  reduce  the  flow  of  cur- 
rent to  a  minimum  value.  This  coil  may 
have  resistance  of  100,000  ohms  and  may 
be  provided  with  two  taps  making  the 
meter  a  double  scale  instrument. 

There  is  essentially  no  difference  in 
the  construction  of  the  ammeter  and 

the  voltmeter  except  the  resistance  of  the  windings  and  the  calibration.  The  coils 
of  the  ammeter  have  relatively  low  resistance  whereas  the  voltmeter  as  already 
mentioned,  has  high  resistance. 

The  windings  of  an  ammeter  may  be  proportioned  to  carry  a  small  amount  of  current, 
but  the  meter  can  be  used  to  measure  very  large  values  by  connecting  its  terminals 
across  an  external  shunt  as  in  Fig.  48. 
This  shunt  consists  of  a  number  of  metal 
strips  of  comparatively  low  resistance 
stretched  between  two  large  copper 
lugs.  A  potential  difference  exists  across 
the  terminals  of  the  shunt  which  causes 
a  certain  amount  of  the  current  to  sub- 
divide and  flow  through  the  meter.  An 
increase  of  current  through  the  shunt 
will  increase  the  flow  of  current  through 
the  instrument,  and  the  meter,  therefore, 

may  be  calibrated  to  read  very  large  Flg"  48-Show'«*  the  Use  of  a  Shunt  Wlth  an  Ammeter" 
values  of  current,  although  but  small  values  pass  through  the  instrument  itself.  Such 
instruments  are  generally  supplied  with  a  certain  length  of  connecting  leads  between 
the  shunt  and  the  instrument.  The  length  of  these  leads  must  not  be  altered  or  the 
calibration  of  the  instrument  will  be  interfered  with.  In  electrical  diagrams,  both  the 
voltmeter  and  the  ammeter  may  be  designated  by  the  symbol  of  Fig.  49. 

One  type  of  meter  for  use  in  alternating  current  circuits  is  shown  in  Fig.  50.  Coil 
VV  is  a  spool  of  wire  with  several  layers  like  the  winding  of  an  electromagnet.  The 
shaft  carrying  the  pointer  P  has  the  semi-cylinder  of  iron  A  surrounded  by  the  brass 
semi-cylinder  B.  Outside  B  is  another  semi-cylinder  of  iron  C.  When  current  is  flow- 
ing through  the  coil  W,  the  semi-cylinder  A  tends  to  move  into  the  unfilled  space  of 
C,  but  is  resisted  by  the  spiral  springs  S. 

If  coil  W  is  wound  with  fine  wire  and  has  a  coil  of  high  resistance  connected  in  series, 
it  is  a  voltmeter,  but  if  W  has  a  coarse  wire  winding,  the  instrument  becomes  an  ammeter. 

(b)  The  wattmeter  is  a  positive  necessity  for  determining  the  power  flowing 


AMMETER 


LOAD 


44 


PRACTICAL  WIRELESS  TELEGRAPHY. 


Fig.    49— Symbol    for    Measuring    Instru- 
ment. 


in  an  alternating  current  circuit  because,  as  already  explained,  the  product  of  the 

volts  multiplied  by  the  amperes  does  not  give  the  true  reading  of  watts.    Due  to 

the  self-induction  of  the  circuit,  the  E.  M.  F.  and  current  do  not  reach  their  maxi- 
mum values  simultaneously.  The  current  in  fact, 
lags  behind  the  impressed  E.  M.  F.,  and  there- 
fore the  product  of  volts  multiplied  by  amperes 
gives  what  is  known  as  an  apparent  reading  of 
watts.  This  lagging  of  the  current  behind  the 
E.  M.  F.  is  known  as  phase  displacement. 

Wattmeters  are  constructed  to  be  independent 
of  phase  displacement  and  they  will  give  true 
readings  of  power  consumption  over  the  range 
for  which  they  are  designed.  The  student  will 
always  recognize  the  wattmeter  by  it  having  four 
binding  posts;  tzvo  very  large  binding  posts  which 
are  connected  in  series  with  the  apparatus  under 
measurement  and  two  small  binding  posts  which 
are  connected  in  shunt  to  the  terminals  of  this 
apparatus. 

The  general  design  for  this  instrument  is  shown  in 
Fig.  51.  Coil  A,  called  the  current  coil,  is  connected 

in  series  with  the  load;   coil  B,  called  the  voltage  coil,  is  connected  in  sliunt  to   the  load 

but  a  large  resistance  R  is  connected  in  series.      The  position  of  the  coil  A  is  fixed,  but 

coil  B  is  mounted  on  bearings  and  fitted  with  a  pointer  P  which  is  held  in  the  zero 

position  by  a  spiral   spring  S.     When   current   is  passed  through  these  two  coils,   two 

magnetic  fields  are  set  up,  which  act  mutually  to  pull  the  movable  coil  B  parallel  to  the  coil  A. 
The   current  in   the  pressure   coil  will  vary  as   the   potential  difference  between   its 

terminals  and  the  current  through  the  series  coil  will  vary  as  the  current  in  the  circuit 

in  which  it  is  inserted.     The  force  acting 

upon  the  movable  coil  will  be  proportional 

to  the  product  of  the  current  and  potential 

difference.      That  is,  the  deflection  of  the 

coil   is    proportional   to   the   power   of   the 

current    flowing    in    the    circuit,    and    the 

scale  of  the  instrument  may  be  calibrated 

directly  in  watts.      In  the  diagram  binding 

posts    C,   C,   are   for  the   current  coil,   and 

posts  V,  V,  are  for  the  pressure  coil. 

(c)  Frequency  meters  are  not  ex- 
tensively supplied  to  commercial  wire- 
less telegraph  sets,  but  one  of  these  in- 
struments is  always  a-  part  of  the  radio 
inspector's  testing  equipment.  The 
Hartmann  &  Braun  meter  is  shown  in 
Fig.  52.  It  has  much  the  appearance  Fig- 

of  a  voltmeter,  and,  like  that  instrument,  it  is  connected  in  shunt  to  the  terminals 
of  the  alternator. 

In  this  diagram  the  single  elongated  magnet  winding  M  has  joined  in  series  with 
it  the  coil  R ;  the  two  terminals  E,  E  are  shunted  across  the  circuit  under  measurement. 

The  soft  iron  piece,  P,  P,  completes  the  magnetic  circuit  for  the  poles  of  the  horse- 
shoe magnet,  N,  S,  N,  S,  etc.  A  number  of  small  vibrating  reeds  are  placed  directly 
in  the  path  of  the  flux  between  the  soft  iron  piece  and  the  poles  of  the  magnet.  Each 
of  these  reeds  have  a  different  period  of  mechanical  vibration  and,  consequently,  are 
only  set  into  vibration  by  the  flux  of  the  magnet  when  it  alternates  at  such  rates  as  to 
correspond  to  the  natural  mechanical  period  of  the  reed. 

Four  permanent  horse-shoe  magnets,  N,  S,  N,  S,  etc.,  keep  the  core  in  a  constant 


ELECTROMAGNETIC   INDUCTION. 


45 


LOAD 


state  of  magnetization,  but  wh'en  alternating  current  flows  through  winding  M,  the 
reed  having  a  natural  period  corresponding  to  the  particular  frequency  of  the  current 
flowing  will  be  set  into  violent  oscillation.  The  scale  reading  corresponding  to  this 

particular  reed  is  the  frequency  of  the  cir- 
cuit under  measurement.  The  instrument 
will  perhaps  be  better  understood  from 
the  side  elevation,  Fig.  52.  Frequency 
meters  of  this  type  are  very  accurate. 

(d)  The  mechanism  of  one  typt 
of  hot  wire  ammeter  is  shown  in  Fig. 
53.  Meters  of  this  construction  are 
particularly  suitable  for  measurement 
of  the  current  at  radio-frequencies.  It 
should  be  self-evident  that  measure- 
ment instruments  having  bobbins  or 
coils  of  wires  are  totally  unsuited  to 
this  work,  first,  because  the  current 
of  high  voltage  and  high  frequency 
would  burn  out  the  coil,  and,  second, 
the  length  of  the  windings  would 
seriously  affect  the  oscillating  prop- 
erties of  the  circuit.  The  self-induc- 

, •     ^      5       ' *c  ^>        tion  of  the  hot  wire  in  a  hot  wire 

l/n  p        meter  is  practically  zero,  and  there- 

fore there  is  no  danger  of  burn-out 
or  short  circuit. 

In  the  diagram,  Fig.  53,  a  steel  plate 
P  is  made  to  pull  against  the  wire  C,  D, 
by  the  spring  S-l.  One  end  of  the  wire 
C,  D  is  attached  to  the  plate  P,  passed 
about  the  pulley  K,  and  again  attached  to  P  at  point  R  where  it  is  insulated.  The  pulley  K 
carries  the  arm  S  with  two  prongs  between  which  is  stretched  a  silk  thread  T  wound 
around  the  shaft  X.  X  carries  the  pointer  P1,  which  moves  over  the  scale. 

The  current  to  be  measured  enters  the  wire  at  point  A  and  leaves  at  the  shaft  K; 
as  the  current  flows,  the  temperature  of  the  wire  rises,  causing  it  to  expand,  but  owing 
to  the  tension  of  S-l,  the  slack  is  taken 
up  at  side  B  and  equilibrium  can  be  re- 
stored only  when  the  pulley  K  rotates 
sufficiently  to  equalize  the  pull  on  the 
spring.  The  rotation  of  K  carries  S 
with  it,  and  S  in  moving  causes  the  silk 
fiber  to  rotate  the  shaft  which  carries 
the  indicating  needle. 

When  the  hot  wire  ammeter  is  used 
to  measure  large  values  of  current,  a 
shunt  must  be  supplied  to  sub-divide  the 
current  flow,  but  an  inductive  shunt, 
even  jvith  one-half  a  turn  of  wire  can- 
not be  employed  to  measure  current  at 
radio-frequencies  because  the  inductance 
of  the  shunt  would  vary  with  each 
change  of  frequency.  Consequently,  hot 
wire  meters  are  constructed  after  the  de- 


Fig.    51 — Mechanism    of    Wattmeter. 


sign  of  Fig.  54,  where  several  resistance 


Fig.   52 — Mechanism   of   Frequency  Meter. 


wires  are  stretched  in  parallel  between  two  large  copper  blocks,  B,  B1.  All  of  these 
wires  are  of  small  diameter,  such  as  No.  36  or  No.  40  B.  &  S.  gauge,  hence  they  offer 
practically  the  same  resistance  to  current  of  radio-frequency  as  to  a  direct  current; 
that  is,  irrespective  of  the  frequency  of  the  current,  the  reading  in  amperes  will  be 
accurate.  One  of  the  wires,  C,  D,  is  selected  to  work  the  indicating  mechanism  in  the 


46 


PRACTICAL   WIRELESS   TELEGRAPHY. 


DAMPING 
DISC 


Fig.    53 — Mechanism    of    Hot    Wire    Ammeter. 


following  manner:  A  wire,  E,  F,  is  attached  to  the  center  of  C,  D,  and  a  silk  thread 
attached  to  it  at  K,  which  is  wound  about  the  shaft  in  such  a  way  as  to  work  against 
the  tension  of  the  spring  S  which,  normally,  would  cause  the  pointer  to  move  to  the 
full  scale  position.  However,  by  means  of  the  thread,  it  is  drawn  to  the  zero  position 
of  the  scale.  When  current  is  flowing  through  C,  D,  the  expansion  of  the  wire  releases 

the  pull  of  the  thread  which  allows  the 
pointer  to  move  across  the  scale  ac- 
cording to  the  degree  of  expansion  and 
the  tension  of  the  spring. 

The  fundamental  principle  of  another 
instrument  for  measuring  radio-frequency 
currents,  used  by  the  Marconi  Company, 
is  shown  in  Fig.  55.  The  complete  details 
for  construction  are  not  opened  for  pub- 
lication, but  briefly,  the  operation  is  as 
follows :  The  current  of  radio-frequency 
Hows  through  several  wires  stretched  be- 
tween blocks  B  and  B1.  A  thermo- 
couple mounted  on  one  of  these  wires 
as  indicated  at  C,  D,  is  heated  by  the 
current  of  radio-frequency  flowing  be- 
tween the  copper  blocks.  As  is  well 
known,  this  junction  sets  up  a  direct 
current  which  flows  through  the 
meter  A.  The  latter  is  a  sensitive  di- 
rect current  'instrument  with  magnetic 
windings,  and  is  calibrated  directly  in 
amperes. 

The  production  of  E.  M.  F.  by  a  thermo-couple  may  be  better  understood  from  Fig. 
56.  If  a  piece  of  bismuth  B  and  antimony  A  be  soldered  or  welded  together  and 
their  ends  connected  to  a  galvanometer,  then  if  the  temperature  of  the  junction  is 
raised  higher  than  the  remainder  of  this  circuit,  a  current  flows  in  the  external  circuit 
from  antimony  to  bismuth.  If  the  junction  is  cooled  below  temperature  of  the  rest 
of  the  circuit,  current  will  flow  in  the  opposite  direction.  There  are  a  number  of 
other  metals  which  when  joined  and  heated  will  produce  a  direct  E.  M.  F.  under 
change  of  temperature,  notably  copper  and  iron,  which  are  frequently  used  as  a  thermo- 
junction. 

50.  Induction  Coil. — \Ye  have  shown  how  an  alternating  current  can  be 
raised  to  a  pressure  of  several 
thousand  volts  by  means  of  the  alter- 
nating current  transformer.  In  a 
somewhat  .similar  manner,,  a  direct 
current  of  low  voltage  can  be  changed 
to  an  alternating  current  of  many 
thousand  volts  by  an  apparatus 
known  as  an  induction  coil. 

Fig.  57  is  a  diagrammatic  sketch  of 
an  induction  coil.  P  is  the  primary 
winding  and  S  the  secondary  winding. 
B  is  a  piece  of  spring  brass  fitted  with 
a  soft  iron  button  that  may  be  attracted 
by  the  core  C.  A  is  an  adjustable  thumb 
screw,  platinum  tipped,  which  makes 
contact  at  C1,  closing  the  circuit  of  the  battery  through  winding  P.  In  practice  sole- 
noid S  is  wound  about  C. 

When  the  battery  circuit  is  closed  at  K,  the  core  becomes  saturated  with  magnetism 
and  attracts  the  armature  B.  B  being  drawn  to  the  end  of  the  iron  core,  the  flow  of 
current  is  broken  at  C1.  Since  the  current  is  now  cut  off  from  P  the  magnetic  field 
disappears  and  the  tension  of  the  spring  causes  the  circuit  to  be  closed  again  at  C1. 
This  process  is  repeated  continuously,  resulting  in  from  30  to  100  breaks  per  second. 


Hot   Wire  Ammeter   with   Internal   Shunt. 


ELECTROMAGNETIC   INDUCTION. 


47 


9 

The  coil  P  generally  has  1  or  2  layers  of  coarse  insulated  copper  wire  of  different 
sizes  (according  to  design)  between  No.  12  and  No.  16,  which  are  thoroughly  insulated  from 
the  core  C.  Winding  P  is  covered  with  an  UILLIAMMFTFR 

insulating  tube  which  supports  winding  S. 
The  secondary  winding  may  have  many  JUNCTION 

thousand  turns  of  very  fine  wire  which 
are  wound  in  the  form  of  pancakes  and 
connected  in  series.  Thus  the  electromo- 
tive force  at  the  terminals  of  winding  S 


Fig.    56 — Production    of   Thermo-Electric  Current. 

may  be  as  great  as  150,000  volts  when 
the  pressure  of  the  current  through  P  is 
20  to  30  volts. 

The  student  should  note  that,  al- 
though the  interrupted  direct  current  in 
winding  P  induces  an  alternating  current 
in  the  winding  S,  the  induced  pressure 
(in  winding  S)  is  considerably  more  in- 
tense at  the  "break"  of  the  primary  cur- 
rent than  at  the  "make."  This  is  due 
to  the  more  rapid  change  of  flux  thread- 
ing through  the  winding  S  when  the  lines 
of  force  collapse  than  when  they  rise;  in 

Fig.  55-Aerial  Ammeter  with  Thermo-Couple.  othef    WQrds?    it    requires    a   longer    period 

to  saturate  the  iron  core  with  lines  of  force  than  to  empty  it.     The  wave  form  of  the  induced 
current  is  shown  in  Fig.  58. 


MAGNETIC 
AMMETER 


B-i 


MINI 


pig<    57 — Fundamental    Diagram    of    Induction   Coil. 


48 


PRACTICAL  WIRELESS  TELEGRAPHY. 


Fig.  58 — Wave  Form  of  Secondary  Induced  Currents. 


(a)  Interrupters.  In  addition  to  the  magnetic  interrupter  shown  in  Fig.  59 
there  are  several  types  of  interrupters  for  induction  coils,  but  since  they  are  seldom 
used  in  modern  wireless  systems,  they  will  not  be  described.  The  electrolytic 

interrupter  is  frequently  employed 
but  not  extensively.  A  diagram  of 
connections  and  a  sketch  is  shown 
in  Fig.  59. 

A  lead  plate  of  convenient  size  is 
immersed  in  a  dilute  solution  of  sul- 
phuric acid  together  with  a  platinum 
electrode  covered  with  a  porcelain 
sleeve.  The  amount  of  platinum  ex- 
posed to  the  action  of  the  acid  is  closely 
regulated  according  to  the  conditions 
of  the  circuit.  When  the  interrupter  is 
connected  in  series  with  the  primary  winding  of  the  induction  coil  the  action  is  as  follows : 
The  current  flowing  through  the  solution  from  platinum  point  to  the  lead  plate  sets  up  an 
electrochemical  action  which  form  a  gas  bubble  on  the  tip  of  the  platinum  electrode.  This 
gas  bubble  insulates  the  platinum  exposed,  thereby  opening  the  primary 'circuit.  The  cur- 
rent flow  having  discontinued,  there  is  nothing  to  sustain  the  gas  bubble,  which  accordingly 
collapses  again,  allowing  the  current  to  flow  through  the  winding,  when  the  above  action 
is  repeated.  A  rather  high  rate  of  in- 
terruption is  thus  secured  which  in- 
duces a  rapidly  pulsating  current  in 
the  secondary  winding.  Interrupters  of 
this  type  frequently  give  1,000  breaks 
of  the  primary  current  per  second  of 
time.  They  will  not  function  well  on 
potentials  less  than  80  volts  direct  cur- 
rent. Fair  results  are  obtained  with 
alternating  current. 

51.  Practical  Electric  Circuits. 
— With'  the  sole  purpose  of  conveying 
to  elementary  students  an  idea  of  the 
wiring  and  certain  fundamental  facts 
surrounding  practical  electrical  circuits, 
a  few  examples  are  herewith  appended. 
In  the  circuit  of  Fig.  60,  direct  current 
at  pressure  of  110  volts  enters  at  the 
terminals  A,  B,  flows  through  the 
fuses  F,  F,  through  the  switch  blades 
D,  D,  and  thence  on  to  the  bank  of 
lamps  assumed  to  consist  of  8  lamps 
connected  in  parallel  or  in  shunt  to  the 
terminals  of  the  power  line.  If  the 
lamps  have  resistance  each  of  220  ohms, 
one-half  ampere  will  pass  through  the 
individual  lamps;  hence,  8  lamps  will 
pass  4  amperes. 

Now  the  voltage  of  this  circuit  may 
be  measured  by  the  voltmeter  V  which 
is  an  instrument  of  high  resistance, 
taking  a  very  small  amount  of  current, 
and  the  current  can  be  measured  by  the 
ammeter  A  which  conversely  is  an  instrument  of  low  resistance.  In  the  problem  cited, 
ammeter  A  should  indicate  current  of  4  amperes,  and  meter  V  pressure  of  110  volts.  The 
power  in  watts  =  4  X  HO  =  440  watts  =  0.44  K.  W. 

(a)  Fuses.  Fuses  are  required  at  points  F,  F,  to  protect  the  generator  connected  to 
terminals  A,  B,  from  accidental  short  circuit  or  overload.  These  consist  of  lead  or  com- 
position alloy  wire  which  melts  when  more  than  a  predetermined  number  of  amperes  pass 
through  them.  In  order  to  protect  the  circuit  of  Fig.  60,  the  fuses  should  have  currem 


o  o  o  o  o 

o  o  o  o  o 

r£j    O  O  O  O  O 

;3      O  O  O  O  O 

o  o  o  o  o 


110  V.  D.C  LEAD'          PLAflNUM 

Fig.     59 — Electrolytic    Interrupter    Connected    to    an 
Induction    Coil. 


ELECTROMAGNETIC    INDUCTION. 


49 


\ 


LAMPS 


carrying  capacity  of  5  or  6  amperes.  Then  if  current  in  excess  of  this  value  is  drawn,  the 
alloy  wire  melts,  completely  cutting  off  the  current. 

In  the  power  circuits  of  the  Marconi  wireless  system,  enclosed  fuses  are  employed  ex- 
clusively.    These   consist   of   a   strip   of   fusible   material   of  the   requisite   current   capacity, 

stretched  between  two  brass  lugs  and  en- 
closed within  a  fiber  cylinder. 

A  second  example  of  a  practical  circuit 
is  shown  in  Fig.  61  where  the  terminals 
A,  B,  connect  to  a  500  volt  direct  current 
generator.  Assume  that  only  110- volt 
incandescent  lamps  were  available  for  light- 
ing purposes;  then  to  prevent  the  lamps 
being  burned  out  by  excessive  voltage  they 
are  connected  in  series.  The  complete  cir- 
cuit will  pass  y?,  ampere  of  current  at  a 
IF  JF  pressure  of  110  volts.  A  voltmeter  con- 

I       I  nected  around  any  of  the  lamps  would  in- 

I        I  dicate  approximately  110  volts.    Should  any 

of  the  lamps  burn  out,  the  circuit  will  be 
Fig    60 — Simple   Power   Circuit.  .       .  ,  i      u          i  i_i«  i_    j       i_ 

broken  and  can  only  be  established  when  a 

new  lamp  is  substituted  for  the  burnt  one.  Since  the  potential  across  the  entire  bank  of 
lamps  is  500  volts,  and  the  current  ^2  ampere,  the  bank  of  lamps  would  require  l/2  X  500  =  250 
watts  or  t/4  K.  W. 

A   third  example  of  a  practical  circuit  is   shown  in  Fig.  62  wherein  alternating  current 
is    transmitted,   let   us    say,    from   a   central    power    station    at   2200  volts    pressure,   passed 

through  the  primary  winding  of  the  trans- 

former  P  which  is  of  relatively  high  re- 
sistance. By  electromagnetic  induction,  a 
current  is  induced  in  the  secondary  wind- 
ing at  a  pressure  of  110  volts  with  a  corre- 
sponding increase  of  current  as  compared 
with  that  flowing  in  the  primary  winding. 
This  current  may  be  employed  convenient- 
ly for  lighting  a  bank  of  lamps  such  as 
indicated  at  L. 

Assume  the  bank  of  lamps  in  this  draw- 
ing   to    consist    of    8    lamps    connected    in 


rig.    61 — Lamps   in    Series   on    500    Volt    Circuits. 


parallel;  they  will  require  4  amperes  of 
current  and  the  fuses  at  F-l  should  have 
capacity  of  5  or  6  amperes.  On  the  other  hand,  the  strength  of  the  current  in  the  primary 
winding  P  is  relatively  low  and  much  smaller  fuses  will  be  employed  to  protect  this  circuit. 
When  electrical  energy  is  transferred  from  the  primary  winding  to  the  secondary  wind- 
ing, certain  losses  due  to  resistance,  heating  of  the  core  and  magjietic  leakage  take 
place,  hence,  the  total  power  flowing  through  P  exceeds  that  taken  from  S.  If  winding  S 


F 

-o~~ o 


ZZOO  VOLTS 
A.C. 


Fig.   62 — Showing  Use   of   Step-Down  Transformer. 

and  the  circuits  associated  therewith  take  440  watts  we  may  assume  a  value  of  500  watts 
for  the  circuit  P.     Since  the  number  of  watts  in  a  given  circuit  =  I  X  E,  then  (assuming 

W                                               500 
no  phase  displacement)   I  =  —  or  for  winding  P,  I  =  =  0.22  amperes,  the  approxi- 

E  2200 

mate  value  of  the  current  flowing  through  winding  P.     A  1  ampere  fuse  would  therefore 
protect  the  primary  circuit  from  overload. 


50 


PRACTICAL  WIRELESS  TELEGRAPHY. 


The  installation  of  all  types  of  a  power  or  radio  equipment  are  made  in  accordance  with 
certain  definite  rules  or  regulations  which  have  been  adopted  by  the  National  Association 
of  Electrical  Inspectors.  These  rules  vary  slightly  in  different  cities  of  the  United  States, 
but  are  generally  in  accordance  with  the  national  code.  The  installation  of  power  or 
wireless  apparatus  should  not  be  undertaken  until  these  rules  have  been  carefully  gone  over. 
The  foregoing  explanations  and  examples  explain  but  partially  the  problems  of  ordinary 
electric  circuits,  but  they  serve  to  show  certain  fundamental  facts  which  should  be  of  some 
value  to  the  elementary  student  of  radio-telegraphy. 


Fig.    62a.— A   High   Power   Radio   Station   in   the   Tropics. 


PART  IV. 
MOTOR  GENERATORS. 

HAND  AND  AUTOMATIC  MOTOR  STARTERS. 

52.  THE  MOTOR  GENERATOR.  53.  FIELD  RHEOSTATS.  54. 
DYNAMOTOR  AND  ROTARY  CONVERTER.  55.  THE  MOTOR 
STARTER.  56.  AUTOMATIC  MOTOR  STARTERS.  57.  PROTECTIVE 
CONDENSERS.  58.  CARE  OF  THE  MOTOR  GENERATOR.  59. 
How  TO  REMOVE  MOTOR  GENERATOR  ARMATURE. 


52.  The  Motor  Generator. — The  required  high  voltage  current  for  the 
operation  of  a  radio-transmitter  is  obtained  from  ( 1 )  a  source  of  alternating  cur* 
rent,  (2)  an  alternating  current  step-up  transformer;  but  practically  all  vessels 
that  have  been  fitted  with  radio  sets  to  date  have  a  direct  current  dynamo  making 
it  necessary  to  install  a  motor  generator  to  obtain  an  A.  C.  source  of  supply. 

A  motor  generator  is  simply  a  motor  and  a  dynamo  coupled  together  on  a  com- 
mon cast  iron  base,  the  motor  being  set  into  rotation  by  direct  current,  the  dynamo 
in  turn  generating  an  alternating  current  of  the  required  voltage  and  frequency. 
Such  machines  may  have  two  or  four  bearings,  two  for  the  motor  armature  and 
two  for  the  generator  armature  or  the  shaft  may  be  strengthened  at  the  center  and 
have  both  armatures  mounted  on  it.  In  this  case  the  shaft  has  two  bearings. 


MOTOR  FIELD 
MOTOR  / 


GEN    FIELD 


A.C. 


Fig.   63 — General   Construction  of  Motor  Generator. 


A  general  outline  of  the  construction  of  a  motor  generator  is  shown  in  Fig.  63  where  a 
direct  current  motor  is  mounted  on  the  left  of  the  cast  iron  base,  the  alternating  current 
generator  on  the  right.  The  motor  receives  direct  current  at  pressure  of  110  volts  (gen- 
erally) and  the  dynamo  generates  alternating  current  at  frequencies  from  60  to  500  cycles 
and  at  voltages  varying  from  110  to  500  volts  according  to  the  design.  The  student 
should  note  from  Fig.  63  that  (1)  the  generator  field  windings  receive  current  from  the 
same  source  as  the  motor,  e.  g.,  in  the  case  of  a  ship  installation,  from  the  ship's  dynamo ; 
(2)  both  the  motor  and  generator  field  windings  are  connected  across  the  D.  C.  power  line. 


52 


PRACTICAL   WIRELESS   TELEGRAPHY. 


For  the  operation  of  radio-transmitters,  a  motor  generator  is  required  that  will  give : 

(1)  Constant  speed  under  variable  load; 

(2)  Constant  alternating  current  voltage  under  variable  load. 

For  the  quenched  spark  transmitter,  a  constant  current  generator  having  a  falling  voltage 
characteristic  under  a  load  is  preferred. 

When  a  motor  generator  is  connected  to  a  wireless  transmitter,  it  is  subjected 
to  an  intermittent  load  following  the  closing  of  the  transmitting  key,  and  there- 


MofcrFJe/d 


Generator  F/'e/c/ 


Mofor/fteo, 


6O-5OO 
Cyc/es 


Fig.  64 — Simple  Shunt  Wound  Motor  Generator. 

fore,  some  means  must  be  provided  whereby  either  a  uniform  alternating  current 
frequency  or  voltage  for  both  can  be  maintained.  In  practice  the  necessary  regu- 
lation is  obtained  by  special  motor  field  and  generator  field  windings.  Hence, 
motor  generators  may  be  classified  with  respect  to  their  windings. 

Three  different  types  are  in  commercial  use : 

(1)  A  shunt  wound  motor  coupled  to  a  simple  alternating  current  generator: 

(2)  A  shunt  wound  motor  coupled  to  an  alternating  current  generator,  having  a 
compound  wound  field: 

(3)  A  motor  with  differentially  compounded  fields  coupled  to  a  simple  alternating 
current  generator. 

An  example  of  type  (1)   is  the  2  K.  W.  500  cycle  Crocker  Wheeler  motor 
generator  used  with  the  Marconi  panel  transmitters ;  of  type  (2)  the  1  K.  W.  60 


MOTOR   GENERATORS. 


53 


cycle  Robbins  &  Meyers  motor  generator;  of  type  (3)  the  2  K.  W.  240  cycle 
motor  generator.  All  three  types  are  in  use  in  the  radio  sets  of  the  American 
Marconi  Company. 

The  fundamental  circuit  of  type  (1)  is  shown  in  Fig.  64;  of  type  (2)  in  Fig. 
65  and  of  type  (3)  in  Fig.  66.  The  student  should  take  careful  note  of  the  position 
of  the  generator  and  motor  rheostats  in  all  three  diagrams  because  in  addition  to 


Gen  l?heo. 


60-5OO 
Cycles 


Fig.  65 — Motor  Generator  with  Compound  Generator  Field  Winding 

the  automatic  regulation  which  these  machines  are  designed  to  give,  initial  adjust- 
ments of  either  voltage  or  frequency  can  be  made  by  the  rheostats.  For  example, 
if  resistance  be  added  at  the  motor  field  rheostat,  the  motor  speeds  up  and,  there- 
fore, increases  the  frequency  of  the  generator.  If  resistance  be  added  at  the  gen- 
erator rheostat,  the  generator  field  current  reduces  and  the  voltage  across  the  arma- 
ture terminals  drops.  If  more  current  is  admitted  to  the  field  coils,  the  voltage  of 
the  armature  increases. 

Explanations  given  in  Part  III  will  cover  the  shunt  wound  motor  generator  of 
Fig.  64.  By  proper  design,  fair  regulation  of  frequency  and  voltage  is  secured 
with  this  type  under  the  conditions  imposed  by  a  wireless  transmitter. 


54 


PRACTICAL  WIRELESS  TELEGRAPHY. 


It  will  be  noted  from  the  diagram  of  Fig.  65,  the  generator  has  two  field  windings,  a 
series  winding  connected  in  series  with  the  motor  armature  and  a  shunt  winding  connected 
directly  across  the  D.  C.  line.  The  windings  are  disposed  on  each  field  pole  of  the  gen- 
erator so  that  the  lines  of  force  generated  by  the  scries  winding  and  those  of  the  shunt 
winding  Hozv  in  the  same  general  direction.  When  a  motor  generator  of  this  type  is  subjected 
to  load,  there  will  be  a  tendency  towards  a  decrease  in  speed,  but  there  will  then  be  an  in- 
crease of  current  through  the  series  winding  (because  it  is  connected  in  series  with  the 


O.C 


Fig.  66 — Circuits  of  Motor  Generator  with  Differential  Motor  Field  Winding. 


armature)  which  has  the  effect  of  increasing  the  strength  of  the  generator  field,  thereby 
maintaining  the  voltage  of  the  alternator  fairly  constant  under  variable  load.  The  motor 
of  this  machine  has  a  simple  shunt  winding  with  a  speed  regulating  rheostat,  connected  in 
series.  The  voltage  of  the  generator  may  be  increased  or  decreased  by  means  of  the 
generator  field  rheostat. 

The  differential  motor  of  Fig.  66  has  been  explained  in  Paragraph  40.  The  principal 
advantage  of  the  motor  generator  in  Fig.  66  is  that  it  maintains  a  uniform  speed  under 
variable  load  which  results  in  a  uniform  frequency  of  current  at  the  terminals  of  the 
alternator. 

A  photograph  of  the  2  K.  W.  500  cycle  Crocker  Wheeler  motor  generator  ap- 
pears in  Fig.  67a  wherein  the  motor  generator  armatures  are  clearly  shown.    The 


MOTOR   GENERATORS. 


55 


MOTOR 
FIELD 


GENERATOR    FIELD 


Fig.   67a— Details  of  Crocker  Wheeler  2  K.  W.  500  Cycle  Motor  Generator. 

generator  has  30  field  poles  and  the  motor  2  field  poles.  The  armature  revolves  at 
2,000  R.  P.  M.,  hence  there  are  33  1-3  revolutions  per  second  which  multiplied 
by  30  (the  number  of  field  poles)  gives  1,000  alternations  of  current  per  second. 
And  since  two  alternations  of  current  constitute  a  cycle,  the  frequency  of  this 
generator  is  500  cycles  per  second.  The  motor  of  this  machine  takes  29  amperes 
at  pressure  of  110  volts  D.  C,  but  the  generator  armature  delivers  alternating 


Fig.   67b — 2  K.   W.  Crocker-Wheeler  Motor  Generator  Assembled. 


56 


PRACTICAL   WIRELESS    TELEGRAPHY. 


ROTOR  OF 
GENERATOR 


GENERATOR 
-  ARMATURE 
MOTOR 
ARMATURE 


Fig.  67c— Details  of    %    K.    W.   500  Cycle  Crocker   Wheeler   Motor  Generator. 

current  at  pressures  varying  between  120  and  380  volts  with  current  output  of  20.8 
amperes.    At  normal  saturation  the  generator  and  the  motor  fields  require  about 

29  X  HO 

2l/2  amperes  each.     The  complete  motor,  therefore,  is  rated  at =  4.2 

746 
horsepower. 

Fig.  67a  shows  the  gen- 
erator and  motor  field  wind- 
ings, the  end  plates  and  the 
oil  gauge  of  the  2  K.  W.  500 
cycle  motor  generator.  The 
machine  completely  assem- 
bled but  with  the  rotary 
disc  discharger  removed  is; 
shown  in  Fig.  67b. 

Details  of  the  y2  K.  W.. 
500  cycle  Crocker  Wheeler 
motor  generator  are  shown! 
in  Fig.  67c  and  the  assem- 
bled machine  in  Fig.  67d. 
It  is  to  be  noted  that  the 
rotor  of  the  generator  is 
composed  of  two  toothed' 
discs  without  windings  and 


Fig.  67d—y2  K.  W.  Motor  Generator  Assembled. 


MOTOR   GENERATORS. 


57 


that  both  the  field  and  armature  windings  are  stationary.  The  rotor  closes  and 
opens  the  magnetic  circuit  between  the  armature  and  field  windings  and  thereby 
induces  current  in  the  armature. 

53.  Field  Rheostats.— A  type  of  field  rheostat  supplied  with  motor  gen- 
erators is  shown  in  Fig.  68.     A  coil  of  composition  resistance  wire  baked  in  a 

heatproof  insulating  cement  is 
mounted  in  a  metal  case.  The 
wire  is  tapped  at  intervals  and  con- 
nected to  brass  studs  over  which  a 
sliding  contact  arm  moves.  An- 
other type  used  by  the  Marconi 
Company  has  a  number  of  turns  of 
bare  resistance  wire  wound  on  a 
slate  base.  A  sliding  contact  mov- 
ing over  the  turns  permits  very 
close  regulation  of  the  flow  of  cur- 
rent in  the  field  winding.  A  rheo- 
stat of  this  type  is  essential  for 
motor  generators  used  with 
quenched  spark  transmitters  which 
require  extremely  close  regulation 
of  the  generator  voltage. 

54.  Dynamotor  and  Rotary 
Converter. — The  dynamotor  and 
the  rotary  converter  are  employed 
occasionally  to  generate  alternat- 
ing current  from  a  D.  C.  source  of 
supply.  The  distinguishing  feature  of  these  machines  is  the  use  of  a  single  arma- 
ture for  both  A.  C.  and  D.  C.,  hence,  but  two  bearings  are  required  and  the  con- 
struction of  the  machine  as  a  whole  is  simplified.  These  machines  also  require 
less  space  to  erect,  but  they  possess  the  marked  disadvantage  of  not  allowing 
full  control  over  the  voltage.  Also  they  are  not  as  efficient  as  the  motor 
generator  when  connected  to  a  radio  transmitter. 


Fig.  68 — Cutler  Hammer  Type  Field  Rheostat. 


110  VOLT  D.C 


A/WW 


Fig.   69 — Fundamental  Circuit  of  Rotary  Converter. 


The  rotary  converter  shown  in  Fig.  69  has  a  single  winding  on  one  armature 
for  both  alternating  and  direct  current,  but  the  dynamotor  of  Fig.  70  has  two 
distinct  windings  (on  the  same  armature)  one  to  rotate  it  as  a  motor  and  the 
other  for  the  production  of  alternating  current. 

Explanation  of  the  circuits  of  the  rotary  converter  of  Fig.  69  follows  :  Direct  current 
from  an  external  source  enters  the  armature  coil  C,  D  through  brushes  A,  B,  and  also 
flows  to  the  shunt  field  windings  (wiring  not  shown)  causing  the  armature  to  revolve  in 
the  usual  way.  Taps  taken  from  this  winding  at  the  commutator  segments  directly  under- 


58 


PRACTICAL  WIRELESS  TELEGRAPHY. 


neath  the  brushes  are  connected  to  the  collector  rings  on  the  opposite  ends  of  the  shaft, 
the  circuit  continuing  through  the  A.  C.  external  circuit  E.  The  voltage  of  the  alternating 
current  will  be  maximum  when  the  taps  to  the  collector  rings  are  underneath  the  brushes 
and  minimum  when  midway  between  the  brushes.  It  is  easily  seen  that  as  C,  D  revolves 
and  attains  the  position  opposite  to  that  in  Fig.  69,  the  current  taken  from  the  collector 


1 10  VOLT  D.C 


WWV 


Fig.  70 — Fundamental  Circuit  of  Dynamotor. 


rings  will  flow  in  the  opposite  direction  and  therefore  as  the  armature  revolves  an  alter- 
nating current  can  be  taken  from  the  armature,  the  frequency  of  which  varies  with 
the  speed.  An  important  point  in  connection  with  this  machine  is  that  if  the  D.  C.  supply 
is  110  volts,  the  A.  C.  voltage  cannot  exceed  70.7  volts.  If  110  volts  is  desired  a  small 
step-up  transformer  must  be  used. 

The  A.  C.  voltage  of  the  converter  may  be  increased  by  increasing  the  speed  of  the 
motor,  but  the  frequency  of  the  current  is  likewise  increased.  The  converter  does  not 
permit  the  nicety  of  control  of  the  voltage  and  the  frequency  as  does  the  motor  generator 
and,  therefore,  it  operates  at  a  disadvantage. 

The  circuit  of  the  dynamotor  is  shown  in  Fig.  70.  Here  the  armature  coils  for  the 
production  of  alternating  current  have  no  connection  with  the  coils  for  direct  current. 
The  two  windings  are  mounted  on  the  same  core  but  in  separate  slots.  The  A.  C.  winding 
can  be  given  the  correct  number  of  turns  so  that  110  volts  A.  C.  may  be  obtained  when 
the  armature  is  connected  to  110  volts  D.  C. 


F-i 


00000 


no  VOLT  D.C 


Fig.  71 — Cutler  Hammer  Starting  Box  Connected  to  a  Simple  Shunt  Wound  Motor. 


A  small  number  of  either  of  the  foregoing  types  of  machines  are  in  use  by  the  Marconi 
Wireless  Telegraph  Company  of  America  but  the  rotary  converter  is  very  popular  abroad. 

55.  The  Motor  Starter. — It  has  been  mentioned  in  Paragraph  40  that 
the  counter  E.  M.  F.  of  a  motor  armature  is  very  low  upon  starting,  and  if  the 
terminals  of  a  motor  were  connected  directly  to  the  source  of  current,  an  exces- 
sive current  would  flow  which  might  injure  the  commutator  or  burn  out  the 
armature  windings  (unless  the  circuit  is  properly  fused).  A  motor  starter  is, 
therefore,  required  to  reduce  the  starting  current  to  a  safe  value. 

A   diagram  of  the  Cutler  Hammer  hand   starter  is  shown  in   Fig.  71.     The  principal 


MOTOR   GENERATORS. 


59 


elements  of  the  starter  are  the  resistance  coils  R-l,  the  small  holding  magnet  M  and  the 
handle  H.  The  coils  of  R-l  are  of  German  silver  wire  or  composition  wire  tapped  at 
certain  intervals  and  connected  to  the  studs  1,  2,  3,  4,  5.  The  circuit  from  the  negative 
side  of  the  D.  C.  power  line  may  be  traced  to  the  L  post  of  the  starter  through  the  handle 
H  which  when  placed  on  the  first  point  of  contact  permits  the  current  to  flow  through  the 
coils  of  R-l  to  the  terminal  A,  thence  to  the  brush  F  of  the  motor  armature.  The  circuit 
continues  through  the  armature  coils  back  to  brush  E  and  to  the  positive  side  of  the  line. 
One  terminal  of  the  field  windings 
F-l  and  F-2  receives  current  at  the 
positive  side  of  the  line  at  brush  E, 
but  the  other  terminal  has  the  field 
rheostat  R-2  connected  in  series, 
also  the  holding  magnet  M  ;  also  this 
circuit  continues  to  the  first  tap  on 
the  resistance  coils  R-l.  Now  as 
the  handle  is  moved  slowly  across 
the  contact  studs  on  the  starter 
current  is  admitted  to  the  motor  arm- 
ature by  small  increments  which  sets 
it  into  rotation,  the  speed  gradually 
increasing  as  the  handle  moves 
toward  the  final  or  full  running  po- 
sition. When  this  position  is  at- 
tained, the  magnet  M  grips  the 
handle  and  holds  it  in  position  until 
it  is  released  by  opening  the  main 
D.  C.  line  switch.  The  diagram 
shows  the  Cutler-Hammer  starter 
connected  to  a  shunt  wound  motor. 


It  is  important  that  a  motor 
be  started  neither  too  rapidly  nor 
too  slowly.  If  the  former  condi- 
tion obtains  the  fuses  in  series 


Fig.    71a — Cutler-Hammer    Hand    Motor    Starter. 


with  the  line  to  the  motor  armature  will  melt,  but  if  the  starting  handle  is  moved 
too  slowly  across  the  contact  studs,  the  internal  resistance  coils  will  overheat  and 
perhaps  burn  out.  The  speed  of  acceleration  of  the  starting  handle  can  usually 
be  gauged  by  observing  the  speed  of  the  motor  armature.  It  should  require  no 
more  than  15  seconds  to  start  the  motors  used  in  connection  with  wireless  tele- 
graph apparatus. 

ARMATURE 


1 10  VOLT  D.C 

Fig.  72 — General  Electric  Company's  Hand  Starter  Connected  to  Shunt  Wound  Motor. 

The  release  magnet  M,  Fig.  71,  serves  to  protect  the  motor  in  case  the  main 
line  circuit  is  disconnected  or  should  by  accident  the  circuit  to  the  motor  field 
windings  be  broken.  In  either  event  the  handle  H  Hies  back  to  the  starting  posi- 
tion by  the  tension  of  a  .spring  attached  to  the  bearing  of  the  handle,  and  thus 
interrupts  the  circuit  to  the  armature. 


60 


PRACTICAL    WIRELESS    TELEGRAPHY. 


The  General  Electric  Company's  hand  starter  differs  slightly  from  the  type  just  de- 
scribed. A  complete  diagram  is  shown  in  Fig.  72  where  the  starter  is  connected  to  a  simple 
shunt  wound  motor.  It  is  to  he  noted  in  this  diagram  that  the  release  magnet  M  is  shunted 
across  the  D.  C.  line,  and  has  a  coil  of  fixed  resistance  R-l  connected  in  series.  In  all 
other  respects,  the  wiring  is  the  same  as  the  first  type  and  the  starter  performs  the  same 


110  VOLT  D.C. 


Fig.   73 — Circuit  of  Automatic  Starter  for  Marconi    l/2   K.   \V.   120  Cycle  Set. 

functions.     If  the  source  of  power  is  cut  off,  magnet  M  releases  the  handle  of  the  starter 
whereupon  it  returns  to  the  zero  position,  breaking  the  circuit  to  the  armature. 

56.  Automatic  Motor  Starters. — It  is  c.ften  essential  to  install  a  motor 
generator  at  a  point  remote  from  the  wireless  cabin  in  order  that  the  noise 
from  its  operation  may  not  interfere  with  the  reception  of  wireless  signals.  In 
instances  of  this  kind  automatic  motor  starters  are  employed,  which  are  con- 
trolled from  a  distant  point  by  pressing  a  small  button  or  closing  a  small  switch. 
Such  starters  possess  the  advantage  that  the  acceleration  of  the  starting  handle  is 
uniform,  and  there  is,  therefore,  no  danger  of  burning  out  the  armature  or  melt- 
ing the  fuses  during  the  starting  of  a  motor. 


1 10  VOLT  D.C 


SHUNT 


[AAAMAMJ  J  *! 


D.C. 
BRAKE  RES. 

VW\AM ' 


Fig.  74— Circuits  of  Automatic  Motor  Starter  for  Marconi   ^  K.  W.  500  Cycle  Set. 

There  are  numerous  types  of  automatic  starters  on  the  electrical  market  but 
only  those  used  in  modern  Marconi  sets  can  be  given  consideration  here. 

The  complete  circuit  of  the  single  step  automatic  starter  is  shown  in  the  diagram  of 
Fig.  73.  It  is  employed  in  connection  with  the  l/2  K.  W.  120  cycle  Panel  transmitters  of 
the  Marconi  Company.  A  single  resistance  unit  R-l  is  connected  in  series  with  the  brushes 


MOTOR   GENERATORS. 


61 


G  and  F  of  the  armature.  The  solenoid  winding  S  connected  in  shunt  to  the  motor  arma- 
ture draws  up  the  plunger  C  which  in  turn  shunts  the  coil  R-l  through  the  contacts  A  and 
B.  When  the  main  D.  C.  line  switch  is  closed,  current  flows  to  the  motor  armature 
through  R-l  and  as  the  counter  E.  M.  F.  of  the  armature  increases,  the  solenoid  winding 
becomes  more  strongly  magnetized,  drawing  up  the  plunger,  which  cuts  out  the  resistance 
coil.  Now  when  the  plunger  of  the  solenoid  is  in  the  full  running  position,  contacts  K  and 
L  are  forcibly  opened  and  the  resistance  unit  R-2  is  connected  in  series  with  the  solenoid 
winding.  This  is  to  prevent  the  magnet  winding  from  overheating  and  consequent  injury. 
The  circuits  of  the  automatic  starter  employed  in  the  ^  K.  W.  500  cycle  transmitting 
sets  of  the  Marconi  Company  are  shown  in  Fig.  74.  When  the  starting  switch  2  is  closed, 
the  solenoid  3  is  connected  in  shunt  to  the  D.  C.  line.  The  flux  from  this  solenoid  attracts 
the  lever  4  making  contact  with  points  5,  thereby  closing  circuit  from  the  D.  C.  line  to  the 
motor  armature  through  the  resistance  coil  6.  Simultaneously  the  solenoid  7  is  connected 
in  shunt  to  the  D.  C.  line  (through  the  lever  4)  which  attracts  the  lever  9  making  contact 
with  point  10,  thus  cutting  the  resistance  6  out  of  the  armature  circuit,  whereupon  the 
motor  is  connected  directly  to  the  main  D.  C.  line.  It  is  apparent  that  the  lever  of  solenoid 
3  opens  and  closes  the  main  power  circuit  while  the  lever  of  solenoid  7  cuts  out  the  re- 
sistance in  series  with  the  motor  armature.  The  solenoids  3  and  7  have  the  resistance  coils 
14  and  8  respectively,  which  are  connected  in  series  with  their  respective  windings  auto- 
matically by  the  levers  4  and  9.  These  resist- 
ances prevent  the  solenoid  winding  from  over- 
heating. 

The  automatic  starter 
also     includes     the     ele- 
ments of  an  electrodyna- 
mic    brake.      When    the 
starting  switch  2  is  open, 
•lever  4  drops  back,  also 
lever  9,  followed  by  con- 
tact being  made  between  points  11  and  12  con- 
necting  the    resistance   coil    13   in    shunt  to   the 

ttipJHBS  *   K^B  motor  armature  and   the   series   winding.      The 

pS§  fcj"4 1  lj|B|  motor    armature    thus    temporarily    becomes    a 

i    jpj  generator  and  owing  to  the  power  expended  in 

tfS&BJ  setting  up  a  current  through  the  resistance   13, 

a  powerful  braking  action  is  set  up  against  the 
armature,  bringing  it  to  a  quick  stop.  The  re- 
sistance coil  15  is  the  motor  field  rheostat,  by 
means  of  which  the  speed  of  the  motor  can  be 
regulated  over  certain  limits. 

The  starting  switch  2  is  usually  one  of  the 
snap  type  placed  conveniently  for  the  wireless 
operator  and  near  to  the  aerial  changeover 
switch.  In  some  installations  the  starting  circuit 
opens  and  closes  through  the  latter  switch,  stop- 
ping the  motor  whenever  the  aerial  switch  is  in 
the  "receiving"  position.  In  case  it  becomes 
necessary  to  install  the  motor  generator  in  the 
operating  room,  it  is  essential  that  the  motor 
stop  immediately  after  the  sending  period,  to 
permit  the  reply  from  a  distant  radio  station  to 
be  deciphered  without  interference. 

The  circuits  of  the  automatic  starter 
supplied  with  the  2  K.  IV.  500  cycle  transmitting  sets  of  the  Marconi  Company  are 
shown  in  Fig.  75a.  In  addition  to  acting  as  a  motor  starter  it  performs  the  func- 
tions of  a  main  line  circuit  breaker  through  the  medium  of  an  overload  relay 
switch.  The  starter  has  three  resistance  units  connected  in  series  with  the  motor 
armature  instead  of  the  single  resistance  unit  described  in  the  two  previous  types. 
It  will  be  observed  from  the  drawing  that  the  field  winding  of  the  motor  is  connected  in 
shunt  with  the  D.  C.  line  through  the  regulating  field  rheostat  23.  As  resistance  is  increased 
at  23,  the  speed  of  the  motor  increases,  and  consequently,  the  frequency  of  the  alternator. 


Fig.   75 — English  Marconi   Company's  one-half 

Kilowatt   Vertical   Type   Motor   Generator   with 

Synchronous  Disc  Discharger  on  Shaft 


62 


PRACTICAL  WIRELESS  TELEGRAPHY. 


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MOTOR   GENERATORS. 


63 


The  generator  field  winding  is  connected  in  shunt  to  the  D.  C.  line  through  the  low  power 
resistance  24  and  the  voltage  regulating  rheostat  25.  The  field  circuit  continues  to  the  con- 
tacts of  the  antenna  switch  62  and  63  through  the  control  switch  26  and  finally  to  contact  5 
of  the  automatic  starter.  By  this  connecion  the  circuit  to  the  generator  field  winding  re- 
mains open  until  the  bar  6  attached  to  the  plunger  A  of  the  automatic  starter  has  touched 
point  5.  When  the  bar  of  the  automatic  starter  makes  contact  with  point  4,  the  D.  C.  arma- 
ture is  connected  directly  to  the  main  D.  C.  line. 

By  increase  of  resistance  at  the  rheostat  25,  the  voltage  of  the  A.  C.  generator  drops  but 
it  may  be  increased  correspondingly  by  the  reduction  of  resistance.  Low  values  of  voltage 
may  be  secured  at  the  terminals  of  the  alternator  by  an  external  fixed  resistance  24  con- 
nected in  series  with  the  generator  rheostat.  This  is  shunted  by  the  switch  indicated  in  the 
drawing. 

The  overload  relay  employed  in  conjunction  with  the  automatic  starter  has  the  magnet 
winding  20,  which  may  be  called  the  tripping  magnet,  and  the  second  magnet  winding  22, 
which  may  be  called  the  holding  magnet.  Winding  20  is  in  series  with  the  D.  C.  armature  on 
the  negative  side  of  the  line.  If  more  than  a  predetermined  number  of  amperes  flow  through 
this  winding,  the  lever  14  is  drawn  up,  breaking  the  circuit  of  the  solenoid  winding  11 
through  the  contacts  13  and  14.  Immediately  afterward  the  circuit  through  winding  22  is 
closed  through  contacts  14  and  21.  This  causes  the  lever  14  to  be  held  in  that  position  until 
either  the  main  D.  C.  line  switch  or  the  starting  switch  17  is  opened. 


SERIES 


FIELD  KA>1 

RHEOS.  ^V\ 

SHUNT       \.    ^ 


60  TO  500 
CYCLES 


110 VOLT  D.C. 


Fig.  76 — Hand  Starter  Connected  to  Motor  Generator  with  Differential  Motor  Winding. 

One  terminal  of  the  solenoid  winding  11  is  connected  to  the  positive  pole  of  the  D.  C.  line 
at  point  12.  The  circuit  continues  through  the  fixed  resistance  9,  shunted  by  the  switch  10, 
through  the  contacts  13  and  14  of  the  overload  relay,  through  contacts  15  and  16  of  the 
antenna  switch,  to  a  terminal  of  the  winding  20,  which  is  of  negative  polarity.  Hence  it  is 
readily  observed  that  the  solenoid  winding  is  connected  in  shunt  to  the  D.  C.  line  when 
either  contacts  15  or  16  or  the  starting  switch  17  is  closed. 

The  switch  10  in  shunt  to  the  resistance  9  is  automatically  opened  by  the  plunger  A  of 
the  automatic  starter  when  it  is  in  the  full  vertical  or  running  position. 

The  resistance  coils  of  the  motor  starter,  connected  in  series  with  the  D.  C.  line  to  the 
armature,  are  progressively  cut  out  of  the  circuit  at  contacts  1,  2,  3  and  4  by  the  bar  6.  When 
the  circuit  to  the  solenoid  11  is  closed,  the  plunger  A  with  the  bar  6  moves  in  a  vertical  posi- 
tion, the  acceleration  being  regulated  by  a  piston  drawn  through  a  vacuum  chamber.  When 
contact  is  made  between  the  bar  6  and  point  1,  the  circuit  to  the  armature  includes  the  entire 
set  of  resistance  coils. 

When  the  circuit  to  the  winding  11  is  interrupted,  either  at  point  17  or  at  the  serial 
switch  contacts  15  and  16,  the  plunger  A  drops  downward  and  through  the  medium  of  con- 
tacts 6  and  7,  the  resistance  coil  8  is  connected  in  shunt  with  the  D.  C.  armature.  At  this 
stage  of  operations  the  momentum  of  the  armature  causes  it  to  become  temporarily  a  D.  C. 
generator  and  current  of  large  value  flows  for  a  few  moments  through  the  resistance  8.  The 
magnetic  field*  thus  set  up  by  the  armature  causes  a  powerful  dragging  action  on  the  field 


64 


PRACTICAL   WIRELESS   TELEGRAPHY. 


C-l 


C-* 


EARTH 


Fig.  77 — Protective  Condensers. 


poles  bringing  the  armature  to  a  quick  stop.  Reviewing  the  foregoing:  When  the  handle 
of  the  type  SH  aerial  changeover  switch  (or  any  other  type)  is  thrown  to  a  transmitting 
position,  the  motor  generator  is  automatically  started,  provided  the  main  D.  C.  line  switch 
is  closed.  It  will  be  brought  to  a  quick  stop  when  the  antenna  switch  is  placed  in  the  receiv- 
ing position,  provided  the  switch  17  remains  open.  If  the  switch  17  is  closed,  the  motor 
generator  can  be  kept  in  a  continuous  state  of  operation  during  the  receiving  period. 

The  speed  of  acceleration  of  the  starter  arm  can  be  very  closely  regulated  by  means  of 
an  adjusting  screw  attached  to  the  bottom  of  the  vacuum  chamber.  It  usually  requires  12 
seconds  to  bring  the  starter  up  to  the  full  running  position. 

When  it  becomes  necessary  to  make  repairs  or  adjustments  to  the  generator  or  the  A.  C. 
power  circuits,  the  generator  field  switch  26  should  be  open.  When  the  motor  generator  is 
to  remain  idle  for  an  indefinite  period,  the  main  D.  C.  line  switch  should  be  opened  to  break 
the  circuit  to  the  field  winding  of  the  motor. 

In  the  diagram  of  Fig.  76  the  complete  circuit  of 
a  differentially  wound  motor  coupled  to  a.  simple 
alternating  current  generator  is  shown,  including  the 
connection  of  the  field  rheostats  and  the  hand  starter. 
The  student  should  give  this  diagram  careful  con- 
sideration as  it  serves  to  show  the  complete  funda- 
mental circuit  of  various  types  of  motor  generators 
in  the  Marconi  service.  This  diagram  should  be 
used  in  answer  to  the  Government  examination 
query  regarding  the  fundamental  circuits  of  the 
motor  generator. 

57.  Protective  Condensers.  When  a  wireless 
telegraph  transmitter  is  in  operation,  a  powerful 
electrostatic  field  is  set  up  in  the  region  about  the 
aerial  wires.  //  the  power  apparatus  is  installed  in 
such  a  manner  that  the  low  voltage  wires  leading  to  the  motor  generator  or  other 
apparatus,  lie  parallel  or  in  proximity  to  the  antenna  wires,  currents  of  very  high 
potential  will  be  induced  in  the  power  wires  which  may  puncture  the  insulation. 
A  path  is  then  afforded  for  the  low  voltage  current  which  may  cause  an  arc,  com- 
pletely short-circuiting  the  windings  of  a  motor  generator.  In  other  words,  this 
induction  sets  up  a  difference  of  potential  between 
the  various  windings  or  between  the  windings  and 
the  frame  of  a  motor  generator  which  may  result 
in  a  disastrous  burnout.  The  low  voltage  wires 
can  be  well  protected  by  installing  them  in  iron  con- 
duit, the  latter  in  turn,  being  thoroughly  connected 
to  the  earth.  The  induced  currents  will  flow  on 
the  surface  of  the  pipe  and  be  neutralized  by  the 
earth  connection,  and  thus  do  no  harm  to  the  power 
wiring.  The  power  wires  of  commercial  radio  in- 
stallations are  either  installed  in  iron  conduit  or  in 
lead-covered  cables,  but  in  addition  to  this  protec- 
tion, protective  devices  known  as  protective  con- 
densers or  protective  resistances  are  employed. 

Protective  condenser  units  consist  of  two  one-half 
microfarad  condensers  connected  in  series  mounted  on  an  insulating  support  as  in  Fig.  77. 
The  middle  connection  is  extended  to  the  earth  and  the  remaining  terminals  connected 
across  the  field  or  armature  windings  of  a  motor  generator  or  between  these  windings  and 
the  frame.  Differences  of  potential  that  may  be  induced  in  such  windings  are  thereby 
neutralized  and  reduced  to  zero  through  the  earth  connection. 

Carbon  or  graphite  rods  of  high  resistance  are  often  employed  for  protective  purposes 
as  shown  in  Fig.  78.  A  single  graphite  rod  having  resistance  of  about  5,000  ohms  is  mounted 
on  an  insulating  support  and  connected  to  earth  at  the  middle  point.  The  two  remaining 
terminals  are  connected  to  the  windings  of  the  motor  generator  to  be  protected.  These  rods 


EARTH 


Fig.    78— Protective    Resistance    Rod. 


MOTOR   GENERATORS. 


65 


— PRO  COND 


have   sufficient   resistance   to  prevent  appreciable   leakage   of  the  low   voltage   current  but 
possess  sufficient  conductivity  to  pass  the  induced  current  of  high  voltage. 

Protective  rods  or  protective  condensers  are  connected  : 

(1)  In  shunt  to  the  motor  armature. 

(2)  In  shunt  to  the  generator  armature. 

(3)  In  shunt  to  the  field  windings  of  the  motor. 

(4)  In  shunt  to  the  field  windings  of  the  generator. 

The  diagram  of  Fig.  79  shows  how  protective  condensers  are  attached  to  the  motor  gen- 
erators in  modern  Marconi  sets.  Condensers  A,  B,  C  and  D  are  of  l/2  or  1  microfarad  ca- 
pacity each.  One  terminal  is  connected  to  a 
binding  post  and  the  other  terminal  to 
the  frame  of  the  motor  generator.  The 
frame  of  the  motor  generator  is  connected 
to  the  earth  at  binding  post  E  or  at  any 
other  convenient  point.  These  condensers 
are  generally  mounted  in  a  containing  rack 
on  the  top  of  the  motor  generator  and  pro- 
tected from  injury  by  a  cast  iron  case. 

In  naval  radio  systems,  fuses  are  con- 
nected in  series  with  the  protective  con- 
densers to  protect  the  power  mains  in  case 
of  puncture  of  the  dielectric. 

58.  Care  of  the  Motor  Generator. 

—When  first  coming  in  contact  with  a 
motor  generator  of  any  type,  the  stu- 
dent should  note  particularly  how  the 
brushes  are  held  in  the  rocker  arm  and 
how  the  connections  are  attached  to 
and  between  the  various  brush  holders. 
He  should  also  note  the  connections 
inside  the  frame  from  the  motor  to 
the  generator.  Particular  observance 
should  be  made  of  the  thrust  bearing 
mounted  on  the  end  of  the  shaft  to 
take  up  the  "end  play."  In  the  case  of 
the  2  K.W  500  cycle  motor  generator,  Fig-  79- 
the  method  of  attaching  the  rotary 
spark  gap  to  the  end  of  the  generator  shaft  should  be  carefully  gofte  over. 

Proper  care  of  the  motor  generator  is  assured  if  the  following  general  rules 
are  observed: 

(1)  Keep  motor  brushes  clean  and  free  from  carbon  dust.  Use  sand  paper  only, 
avoid  emery  cloth. 

(2)  Clean  commutator  occasionally  with  a  fine  grade  of  sand  paper. 

(3)  Oil  bearings  frequently.    Open  up  petcocks  occasionally,  to  assure  that  oil 
container  has  the  necessary  supply. 

(4)  Make  sure  that  all  petcock  valves  are  tight  so  that  they  will  not  loosen  by 
vibration. 

(5)  Wipe  off  frame  of  motor  generator,  brush  holders,  and  rocker  arm  occa- 
sionally to  prevent  accumulation  of  carbon  dust  and  grease. 

(6)  Do  not  overspeed  motor.  Normal  speed  can  be  observed  by  the  reading  of 
the  frequency  meter  or  by  applying  a  speed  indicator  to  the  end  of  the 
motor  generator  shaft.   Observe  either  wattmeter  or  ammeter  occasionally 
to  insure  that  the  normal  load  of  the  generator  is  not  exceeded. 

(7)  When  removing  armature  from  motor  generator,  it  is  generally  more  con- 
venient to  take  off  the  generator  end  plate. 

(8)  Be  careful  not  to  injure  commutator  by  scraping  against  the  field  poles. 

(9)  See  that  protective  condensers  are  at  all  times  properly  connected. 

(10)  Punctured  condensers  should  be  removed  or  disconnected  from  the  circuit. 


H^rc^f^rCGelTteorrS,are  C°" 


66 


PRACTICAL  WIRELESS  TELEGRAPHY. 


(11)  In  the  case  of  the  2  K.  W.  500  cycle  motor  generator,  adjust  overload 
relay  for  35  amperes. 

(12)  If  a  single  resistance  coil  in  either  the  hand  starter  or  the  automatic  starter 
burns  out,  close  the  circuit  by  placing  a  jumper  around  the  burned  out 
portion. 

(13)  If  field  rheostat  burns  out,  close  the  circuit  by  a  jumper.    If  burned  beyond 
repair,  substitute  3  or  4  16  C.  P.  lamps,  connected  in  parallel. 

(14)  Tighten  up  all  connections  frequently.    These  should  be  gone  over  once 
per  month. 

59.  How  to  Remove  Motor  Generator  Armature. — In  case  it  becomes 
necessary  to  remove  the  armature  of  the  2  K.  W.  500  cycle  motor  generator  for  the  purpose 
of  repairs,  it  is  necessary  first  to  remove  the  casing  of  the  spark  gap.  Follow  this  by  taking 


Fig.  79a — 1   K.   W.,  60-cycle  Motor-generator   (Two  Bearing  L'niu. 

out  the  wedge-shaped  key  in  the  end  of  the  generator  shaft.  If  the  rotary  disc  is  given  a 
slight  tap  with  the  hammer,  the  key  will  be  released  and  the  disc  may  be  removed  from  the 
shaft.  After  this,  the  bearing  bracket  can  be  removed  from  the  generator  end.  The  brushes 
should  then  be  removed  from  the  commutator  and  the  collector  rings.  After  these  operations 
have  been  gone  through,  the  armature  can  be  pulled  out  and  a  new  one  inserted,  if  necessary. 
When  the  armature  is  replaced,  the  oil  rings  should  .be  held  up  to  permit  the  shaft  to  pass 
through  the  bearings.  Care  should  be  taken  to'  see  that  the  oil  rings  are  working  properly 
and  that  the  bearings  are  thoroughly  oiled  for  the  initial  test.  Before  starting  the  motor 
generator  careful  inspection  should  be  made  to  see  that  all  parts  are  properly  secured  and  in 
working  order. 

It  should  be  noted  that  the  mica  of  the  commutator  of  this  machine  is  undercut  about  1-32 
inch,  and  before  it  gets  flush  with  the  commutator  bars,  the  mica  should  be  cut  out  again. 


PART  V. 

STORAGE  BATTERIES  AND  CHARGING 

CIRCUITS. 

60.  THE  NECESSITY  FOR  A  STORAGE  BATTERY  IN  A  RADIO  INSTAL- 
LATION. 61.  GENERAL  CONSTRUCTION  AND  ACTION.  62.  THE 
CHARGING  PROCESS.  63.  THE  FUNDAMENTAL  ACTIONS  OF  A 
STORAGE  CELL.  64.  THE  ELECTROLYTE.  65.  THE  HYDRO- 
METER. 66.  How  THE  CAPACITY  OF  A  STORAGE  CELL  Is  RATED. 

67.  FUNDAMENTAL   FACTS   CONCERNING  THE    STORAGE   CELL. 

68.  How  TO  CHARGE  A  STORAGE  CELL.     69.  How  TO  DETERMINE 
THE  VALUE  OF  THE  CHARGING  RESISTANCE.     70.  LAMP  BANK 
RESISTANCE.     71.  THE  USE  OF  THE  AMMETER  AND  THE  UNDER- 
LOAD   CIRCUIT    BREAKER.     72.  THE    AMPERE    HOUR    METER. 
73.  OVERCHARGE.     74.  How  TO  CHARGE  A  BATTERY  WHEN  THE 
VOLTAGE    EXCEEDS    THAT    OF    THE    CHARGING    GENERATOR. 
75.  How  TO  DETERMINE  THE  POLARITY  OF  THE  CHARGING  GENE- 
RATOR.    76.  DETERMINATION   OF  THE  STATE  OF  CHARGE  AND 
DISCHARGE  OF  A  BATTERY.     77.  KEEPING  THE  LEVEL  OF  THE 
ELECTROLYTE.     78.  PROTECTING  THE  CE^LS  FROM  ACID  SPRAY. 
79.  GENERAL  INSTRUCTIONS  FOR  THE  PORTABLE  CHLORIDE  TYPE 
OF   ACCUMULATORS.     80.  GENERAL    OPERATING    INSTRUCTIONS 
FOR  THE   EXIDE  CELL.     81.  TlIE   EDISON    STORAGE   BATTERY. 
82.  THE  CHARGE  AND  DISCHARGE  OF  THE  EDISON  CELL. 

60.  The  Necessity  for  a  Storage  Battery  in  a  Radio  Installation. — The 
International  Radio-Telegraphic  regulations  require  that  an  auxiliary  source  of 
direct  or  alternating  current  be  available  for  operation  of  the  motor  generator  or 
a  low  powered  emergency  transmitter  in  case  of  an  accident  to  a  vessel  at  sea 
which  might  put  out  of  action  the  ship's  generator. 

The  United  States  regulations  (Act  of  August  13,  1912)  require  that  the 
auxiliary  radio  transmitter  be  capable  of  transmitting  to  a  distance  of  100  miles. 
A  small  A.  C.  or  D.  C.  generator  operated  by  a  gasoline  or  oil  engine  is  permissi- 
ble as  a  source  of  current  supply  under  the  United  States  statute,  but  the  general 
custom  is  to  employ  a  battery  of  storage  cells  for  direct  operation  of  the  motor 
generator  or  emergency  transmitter. 

Two  general  types  of  storage  cells  are  used  in  connection  with  emergency 
transmitters — the  lead  plate,  sulphuric  acid  cell  such  as  the  "chloride"  and  "exide" 
types  manufactured  by  the  Electric  Storage  Battery  Company  and  the  Edison 
nickel  iron-alkali  cell.  Certain  fundamental  facts  concerning  the  charge  and  dis- 
charge of  storage  cells  and  the  standard  circuits  for  their  use  will  now  be  con- 
sidered, which  is  to  be  followed  in  another  chapter  (Part  X)  by  a  complete  de- 
scription of  the  charging  panels  employed  in  commercial  marine  radio  instal- 
lations. 


68  PRACTICAL   WIRELESS   TELEGRAPHY. 

61.  General  Construction  and  Action. — It  is  not  really  electricity  which 
is  stored  up  in  a  storage  cell,  but  the  flow  of  current  from  a  direct  current  dynamo 
through  the  cell  from  plate  to  plate  performs  a  certain  amount  of  chemical  ivork. 
Whenever  required  this  stored  up  chemical  energy  can  be  released  in  the  form  of 
an  electric  current  which  will  pass  from  plate  to  plate  through  an  external  circuit. 
The  common  type  of  lead  cell  comprises  a  set  of  prepared  lead  plates  immersed 
in  a  dilute  solution  of  sulphuric  acid,  but  a  certain  electrochemical  process  known 
as  "charging"  must  be  gone  through  in  order  that  the  cell  may  deliver  a  current 
of  electricity. 

There  are  two  general  methods  by  which  the  lead  plates  for  a  storage  cell 
may  be  prepared : 

(1)  A  paste  of  litharge  or  oxide  of  lead  mixed  with  a  dilute  solution  of  sulphuric 
acid  may  be  applied  to  perforations  in  a  lead  grid,  and  then  by  means  of  a 
current  of  electricity  and  a  suitable  electrolyte  the  surface  of  some  of  these 
plates  may  be  coated  with  peroxide  of  lead  while  other  plates  become  simply 
spongy. 

(2)  Large  lead  plates  may  be  immersed  in  a  certain  electrolyte  and  connected 
to  the  terminals  of  a  dynamo.    By  repeated  charge  and  discharge,  some  of 
the  plates  may  be  coated  with  peroxide  of  lead  while  the  others  simply  be- 
come spongy. 

Reviewing  the  development  of  the  storage  battery,  we  find: 

(1)  That  the  earlier  types  of  storage  cells  comprised  two  lead  plates  immersed 
in  a  dilute  solution  of  sulphuric  acid.   The  terminals  of  the  plates  were  con- 
nected to  a  direct  current  dynamo  for  a  period  of  several  weeks.  By  repeated 
charges  and  discharges,  the  surface  of  the  plates  received  a  coating  of 
so-called  "active  material." 

(2)  Later  it  was  determined  that  the  formation  of  the  plates  could  be  hastened 
by  chemical  means  previous  to  the  charging  process  and  the  manufacture 
of  the  plates  was  accordingly  cheapened. 

(3)  In  certain  types  of  present  day  cells,  for  instance,  the  exide  lead  cell,  the 
active  material  is  applied  to  the  plates  mechanically,  in  the  form  of  a  paste. 

62.  The  Charging  Process. — In  general  the  charging  process  of  a  storage 
cell  is  as  follows : 

When  two  ordinary  lead  plates  or  sets  of  plates  are  placed  in  a  dilute  solution  of  sulphuric 
acid  of  the  correct  proportion  and  a  direct  current  of  electricity  from  a  dynamo  passed  from 
one  plate  through  the  solution  to  the  other,  the  resulting  chemical  decomposition  deposits  a 
coating  of  peroxide  of  lead  on  one  plate  while  the  other  plate  becomes  gray  and  spongy  or 
porous.  When  one  set  of  plates  is  fairly  well  coated  with  lead  peroxide  and  the  other  set 
becomes  spongy,  the  cell  is  said  to  be  "charged."  If  the  terminals  of  these  plates  are  now 
joined  together  by  a  conductor  (the  charging  generator  having  been  disconnected),  a  cur- 
rent of  electricity  will  flow  from  plate  to  plate. 

The  plate  coated  with  lead  peroxide  is  known  as  the  positive  plate  of  the  storage  cell  and 
the  other,  the  negative  plate.  When  joined  together  by  a  copper  conductor,  current  flows  in 
the  external  circuit  from  the  positive  to  the  negative  plate  and  the  resultant  chemical  change 
undoes  the  work  of  charging,  e.  g.,  part  of  the  peroxide  of  lead  of  the  positive  plate  and  the 
active  material  on  the  negative  plate  is  converted  to  lead  sulphate  which  covers  the  surface. 
When  the  plates  are  fairly  well  coated  with  sulphate,  the  cell  is  said  to  be  discharged.  In 
order  that  current  may  be  drawn  from  the  cell  again,  the  plates  must  be  connected  to  a 
source  of  direct  current  and  the  charging  process  repeated. 

The  processes  involved  in  the  charge  and  discharge  of  storage  cells  may  be 
better  understood  from  the  explanation  given  by  the  Electric  Storage  Battery 
Company  in  the  following  paragraph. 

63.  The  Fundamental  Actions  of  a  Storage  Cell. — When  a  lead  storage 
cell  is  put  on  discharge,  the  current  is  produced  by  the  acid  of  the  solution  going  into  and 
combining  with  the  porous  part  of  the  plate  called  "active  material."    In  the  positive  plate,  as 
stated  before,  the  active  material  is  lead  peroxide  and  in  the  negative  plate  it  is  metallic  lead 
in  a  spongy  form. 


STORAGE   BATTERIES   AND   CHARGING   CIRCUITS. 


69 


When  the  sulphuric  acid  in  the  solution  combines  with  the  lead  in  the  active  material,  a 
compound  known  as  "lead  sulphate"  is  formed. 

As  the  discharge  progresses,  the  solution  becomes  weaker  by  the  amount  of  the  acid  used 
in  the  plate,  which  incidentally  produces  the  compound  of  acid  and  lead  called  "lead  sulphate." 
This  sulphate  continues  to  increase  in  quantity  and  bulk,  thereby  filling  the  pores  of  the  plate. 
As  the  pores  of  the  plate  become  filled  with  sulphate,  the  free  circulation  of  acid  in  the  plate 
is  retarded,  and  since  the  acid  cannot  then  get  into  the  plate  fast  enough  to  maintain  the 
normal  action,  the  battery  becomes  less  active,  as  is  indicated  by  a  rapid  drop  in  voltage. 

During  the  charging  period  direct  current  must  pass  through  the  cells  in  the  direction 
opposite  to  that  of  discharge.  This  current  will  reverse  the  action  which  took  place  in  the 
cells  during  discharge.  It  will  be  remembered  that  during  discharge  the  acid  of  the  solution 
went  in  and  combined  with  the  active  material,  filling  its  pores  with  sulphate  and  causing  the 
solution  to  become  weaker.  Reversing  the  current  through  the  sulphate  in  the  plate  restores 
the  active  material  to  its  original  condition  and  returns  the  acid  to  the  solution.  Thus,  dur- 
ing charge,  the  solution  gradually  becomes  stronger  as  the  sulphate  in  the  plate  decreases, 
until  no  more  sulphate  remains  and  all  the  acid  has  been  returned  to  the  solution,  when  it 
will  be  of  the  same  strength  as  before  the  discharge  and  the  same  acid  will  be  ready  to  be 
used  over  again  during  the  next  discharge.  Since  there  is  no  loss  of  acid  by  this  process, 
none  should  ever  be  added  to  the  solution. 

The  whole  object  of  charging  therefore  is  to  drive  from  the  plates  the  acid 
which  if  now  absorbed  by  them  during  discharge. 

64.  The  Electrolyte. — The  liquid  in  a  storage  cell  is  known  as  the  elec- 


B 


ELECTROLYTE 


trolyte  which  in  the  case  of  the  lead  cell  is  a  20  per  cent, 
solution  of  sulphuric  acid.  It  is  important  to  have  the 
electrolyte  of  the  right  strength  or  the  cell  will  not  func- 
tion properly.  The  strength  of  the  electrolyte  or  the 
proportion  of  the  acid  to  water  is  expressed  in  terms  of 
specific  gravity. 

The  specific  gravity  of  a  compounded  solution  is  a 
measure  of  its  density  or  weight  as  compared  to  that  of 
chemically  pure  water.  If  water  be  taken  as  unity  (or  1) 
it  is  found  that  certain  compounded  solutions  of  acid, 
etc.,  are  heavier  than  water  by  a  certain  amount.  Thus 
the  specific  gravity  of  the  electrolyte  of  one  type  of  lead 
plate  stofage  cell  is  approximately  1.215,  meaning  that  if 
a  cubic  centimeter  of  water  weighs  one  gram,  one  cubic 
centimeter  of  the  electrolyte  weighs  1.215  grams.  It  is, 
therefore,  evident  that  the  greater  the  proportion  of  the 
acid  in  the  electrolyte  of  a  storage  cell,  the  higher  will  be 
the  reading  of  the  specific  gravity. 

65.  The    Hydrometer. — The    gravity    of    the    solu- 
tion of  a  storage  cell  is  measured  by  an  instrument  known  as  a 
hydrometer,  B.  sketch  of  which  is  shown  in  Fig.  80.     The  long 
glass  rod  A-B  has  the  bulb  at  one  end  loaded  with  shot  or  mer- 
cury.   When  dropped  into  a  solution  of  acid  it  sinks  to  a  certain 
depth,  depending  on  the  weight  of  the  liquid.     If  placed  in  chem- 
ically pure  water,  the  hydrometer  would  sink  to  the  bottom  or  at 
least  below  the  level  of  the  liquid,  but  if  it  is  placed  in  a  dilute 
solution  of  sulphuric  acid,  a  part  of  the  tube  protrudes  above  the 
surface.     The  reading  of  the  hydrometer  scale  at  the  surface  of 
the  solution  is  a  measure  of  the  specific  gravity.     The  specific 
gravity  of  the  chloride  portable  storage  cell  varies  between  1.205 
and  1.215  and  for  the  various  exide  cells  between  1.280  and  1.300. 

66.  How  the  Capacity  of  a  Storage  Cell   Is  Rated. — The  capacity  of  a 
storage  cell   is   rated  in  amperehours.     The  amperehour   is   the  unit  employed 
to   express   the   equivalent  quantity   of   current   represented  by   current   of   one 
ampere   flowing   through   a   given   circuit   for  an   hour   of   time.      The   storage 
cells  used  with  commercial   radio  sets  are   rated  at  60  to  224  ampere  hours 


WA 


Fig.    80 — The    Hydrometer. 


70 


PRACTICAL   WIRELESS   TELEGRAPHY. 


capacity  according  to  the  power  of  the  particular  installation.  The  cells  of 
larger  capacity  are  used  for  auxiliary  lighting  service  as  well  as  for  operation 
of  the  radio-transmitter. 

A  clearer  interpretation  of  the  practical  application  of  the  term  amperehour  as  applied 
to  the  rating  of  storage  cells  may  be  obtained  from  the  following  explanation.  For  ex- 
ample, the  chloride  type  of  cell  is  rated  on  an  8  hour  basis  and  the  exide  cell  on  a  4  hour 
basis.  Thus  if  the  chloride  cell  is  rated  as  having  capacity  of  60  amperehours,  the  normal 
discharge  rate  is  determined  by  simply  dividing  the  rating  60  by  8  or  7^  amperes,  which  is 
the  normal  current  is  amperes  to  be  taken  from  the  cell  during  discharge,  and  in  the  same 
manner  the  normal  discharge  rate  of  a  60  amperehour  exide  cell  is  found  to  be  15  amperes. 
The  normal  discharge  rate  of  the  chloride  lead  cells  (in  use  in  the  Marconi  Service)  is  also 
the  normal  charging  rate,  but  different  charging  rates  are  given  for  the  exide  lead  cell 
according  to  the  rated  capacity  of  the  battery.  Generally  at  the  beginning,  the  batteries  are 
charged  at  a  heavy  rate,  but  as  the  charge  progresses  the  charging  current  is  reduced. 

VOLTAGE 
REGULATOR 


Fig.  81 — Simple  Charging  Circuit. 

67.     Fundamental  Facts  Concerning  the  Storage  Cell.   The  student  should 
bear  in  mind  certain  facts  concerning  the  storage  cell : 

(1)  It  has  low  internal  resistance  and  therefore  delivers  a  very  strong  current. 

(2)  The  flow  of  current  is  more  lasting  than  in  the  ordinary  types  of  primary 
cells. 

(3)  The  "charging"  process  requires  considerable  time — a  matter  of  hours  to 
convert  the  surface  of  the  plates  to  so-called  "active  material." 

(4)  During   the   charging   period,    a    stated   number   of   amperes   must   pass 
through  the  cell — the  actual  value  being  designated  by  the  maker. 

(5)  A  normal  rate  of  discharge  is  given  by  the  manufacturers  which  must  not 
be  exceeded  except  in  special  types. 

(6)  The  fully  charged  voltage   of  the  lead  cell  averages  2.1  volts  on  open 
circuit,  values  as  high  as  2.5  or  2.6  volts  being  obtained  with  the  charging 
current  flowing. 

(7)  The  lead  cell  is  said  to  be  discharged  when  the  voltage  of  the  individual 
cell  falls  to  1.7  or  to  1.8  volts  provided  the  reading  is  taken  at  normal 
rates  of  discharge  current. 

(8)  The  fully  charged  voltage  of  the  Edison  cell  is  1.2  volts. 

(9)  The  Edison  cell  is  said  to  be  discharged  when  the  voltage  of  the  cell,  at 
normal  discharge  rates,  falls  to  .9  volts. 

(10)  The  Edison  cell  and  the  lead  cell  differ  both  in  material,  general  con- 
struction, and  electrolyte. 

68.  How  to  Charge  a  Storage  Cell. — A  storage  battery  is  "charged" 
by  connecting  the  positive  terminal  of  the  battery  to  the  positive  terminal  of  a 
direct  current  dynamo,  and  the  negative  terminal  of  the  battery  to  the  negative 
terminal  of  the  dynamo,  but  a  resistance  or  regulating  rheostat  must  be  connected 
in  series  with  the  charging  circuit,  otherwise,  an  excess  of  current  will  flow  and 
the  battery  and  perhaps  the  generator  put  out  of  commission.  This  is  due  to  the 
fact  that  the  storage  cell  possesses  very  low  internal  resistance. 

An  elementary  charging  circuit  is  shown  in  Fig.  81  where  the  brushes  of  the  generator 


STORAGE    BATTERIES   AND   CHARGING    CIRCUITS. 


71 


E,  E,  are  connected  to  the  positive  and  the  negative  terminals  of  a  30  volt  battery  with  a 
regulating  resistance  R  connected  in  series.  The  resistance  of  R  varies  as  the  normal  charg- 
ing rate  of  the  particular  battery  under  charge,  from  5  amperes  in  the  smaller  types  of  cells 
to  50  or  100  amperes  in  the  larger  types.  The  resistance  coil  may  be  of  fixed  or  variable 
value.  It  is  sometimes  fitted  with  a  single  blade  switch,  permitting  a  portion  of  the  coil  to 
be  short  circuited,  to  provide  two  values  of  charging  current.  It  is  made  up  of  a  resistance 
wire  alloy,  constructed  to  withstand  continuous  flow  of  current  at  the  normal  rating  of  the  bat- 
tery without  overheating. 

When  the  circuit  from  the  charging-  generator  is  closed,  current  flows  from 
plate  to  plate  through  the  electrolyte  until  the  surface  is  converted  to  "active 
material,"  the  process  requiring  several  hours,  according  to  the  degree  to  which 
the  cell  has  been  discharged.  In  any  case  the  charging  should  continue  until 
there  is  no  further  rise  in  either  the  specific  gravity  or  the  voltage. 


mm 


LAMPS 


Fig.    82 — Diagram    of    Simple    Charging    Circuit    Including    Lamp    P>ank    "Resistance    and    Underload    Circuit 

Breaker. 

It  should  be  kept  in  mind  that  THE  VOLTAGE  OE  THE  CHARGING 
DYNAMO  MUST  ALWAYS  EXCEED  THE  MAXIMUM  VOLTAGE  OE 
THE  STORAGE  BATTERY  BECAUSE  THE  VOLTAGE  OF  THE  BAT- 
TERY EXERTS  A  BACK  PRESSURE  OR  COUNTER  E.  M.  F.  ON  THE 
CHARGING  SOURCE  AND  IF  THE  VOLTAGE  OF  THE  DYNAMO  IS 
LESS  THAN  THAT  OF  THE  BATTERY  THE  LATTER  WILL  NOT  BE 
CHARGED. 

69.  How  to  Determine  the  Value  of  the  Charging  Resistance. — The  resist- 
ance of  the  series  charging  rheostat  for  a  given  battery  may  be  determined  by 
Ohm's  law.  Assume  that  the  battery,  Fig.  81,  has,  when  fully  charged,  a  voltage 
of  30  volts,  that  the  normal  charging  rate  as  given  by  the  manufacturer  is  6 
amperes  and  the  voltage  of  the  generator  110  volts;  then  the  battery  exerts  a 
back  pressure  of  30  volts  on  the  generator  and  the  net  effective  voltage  is  110  - 
30  volts  or  70  volts. 

For  ordinary  direct  current  circuits  according  to  Ohm's  law, 

R=^ 

I 
hut  in  this  case  the  value  of  R  is  determined  by 

E-e 

R  =- 
I 
where  E  —  the  voltage  of  the  charging  generator, 

e    -  the  fully  charged  voltage  of  the  battery, 
I    -  the  normal,  charging  current  as  given  by  the  manufacture. 
Then  in  the  example  cited, 
110  —  30 
R =11.6  ohms. 


72 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Fig.  83 — Motor  of  Ampere  Hour  Meter, 


70.  Lamp  Bank  Resistance. — A  bank  of  16  or  32  C.  P.  lamps  are  frequently 
employed  to  adjust  the  flow  of  current  through  a  battery  undergoing  charge.     A  circuit  of 
this  type  is  indicated  in  Fig.  82.     Since  the  resistance  of  the  battery  is  small  compared  to 
that  of  the  lamps,  the  flow  of  current  through  the  cells  is  governed  by  resistance  of  the 
lamps. 

A  16  C.  P.  incandescent  lamp  having  a  carbon  filament  passes  y2  ampere  under  an  E.  M.  F. 
of  110  volts  hence  15  such  lamps  connected  in  parallel  will  pass  7l/2,  amperes,  about  the 
correct  value  for  the  smaller  types  of  lead  cells.  The  strength  of  the  charging  current  can 
be  regulated  in  small  steps  by  turning  on  or  off  a  certain  number  of  lamps. 

71.  The  Use  of  the  Ammeter  and  the  Underload  Circuit  Breaker. — To  per- 
mit the  charging  current  to  be  carefully  regulated,  the  charging  circuit  should 
include  an  ammeter  as  at  A,  Fig.  82. 

A   very    important   protective   mechanism    for   a    storage   battery   charging   circuit   is   an 

underload  circuit  breaker.  The  function 
of  this  breaker  is  to  open  the  charging  line 
when  the  voltage  of  the  charging  generator 
falls  below  the  voltage  of  the  battery.  The 
battery  is  thus  protected  from  discharge  in 
case  the  charging  generator  is  stopped 
without  first  disconnecting  the  batteries.  A 
very  simple  type  of  underload  breaker  used 
on  the  small  panel  sets  of  the  Marconi 
Company  is  shown  in  Fig.  82.  A  solenoid 
winding  W  is  connected  in  series  with  the 
charging  circuit  and  carries  the  full  value 
of  the  charging  current,  the  resulting  flux 
acting  upon  the  plunger  P  which  carries 
the  crossbar  and  the  copper  contacts  S-l 
and  S-2. 

When  pushed  up  by  hand,  plunger  P 
closes  the  circuit  from  the  charging  generator  to  the  battery.  The  winding  W  is  thus  magne- 
tized and  holds  the  iron  plunger  in  this  position  until  the  charging  circuit  is  actually  interrupted 
or  the  voltage  of  the  generator  falls  below  the  battery.  In  event  of  either  happening,  the 
plunger  drops  downward,  breaking  the  circuit. 

72.  The  Ampere-Hour  Meter. 
A    particularly    useful    instrument 
for  denoting  the  state  of  "charge" 
or  "discharge"  of  a  storage  battery 

is  the  ampere-hour  meter,  which  in         CURRENT 
essence  is   a   specially   constructed 
small    motor    connected    in    series 
with  the  charge  and  discharge  of  a 
storage  battery. 

The  motor  is  geared  to  a  pointer 

moving     Over     a     dial     Calibrated     in  FI'R.    84— Sketch    Showing    the    Direction    of    Fluxes    in 

ampere  hours,   and   therefore  per-  Ampere  Hour  Meter  Motor' 

mits  a  direct  reading  of  the  quantity  of  current  flowing  during  the  discharge  of  a 

storage  cell. 

A  diagram  of  the  motor  of  the  Sangamo  ampere  hour  meter  is  given  in  Fig.  83. 
It  consists  of  a  copper  disc  D,  floated  in  a  pan  of  mercury  between  the  poles  of  a 
permanent  magnet  to  which  current  is  lead  by  the  contacts  C-l,  C-2.  The  current  of 
electricity  enters  the  contact  C-l,  passes  through  the  comparatively  high  resistance 
mercury  H,  to  the  edge  of  the  low  resistance  copper  disc  P,  across  the  disc  to  the 
mercury  H,  and  out  at  contact  C-2.  The  magnetic  flux  cuts  across  the  disc  on  each 
side  from  N  to  S,  making  a  complete  circuit  through  M-l  and  M-2.  The  relative 
directions  of  the  magnetic  flux  and  the  current  of  electricity  as  well  as  the  resulting 
motion  are  shown  in  Fig.  84. 

According  to  the  laws  of  electromagnetic  induction,  if  a  current-carrying  conductor  cuts 
a  magnetic  field  at  right  angles,  a  force  is  exerted  upon  the  conductor  tending  to  push  it 


CURRENT 


STORAGE   BATTERIES   AND   CHARGING   CIRCUITS. 


73 


at  right  angles  to  both  the  current  and  the  flux;  hence  the  disc  of  the  meter  revolves  at  a 
uniform  rate. 

When  fitted  with  an  eddy  current  damper  or  generator  which  requires  a  driving  force 
directly  proportional  to  the  speed  of  rotation,  the  mercury  motor  generator  becomes  a 
meter.  The  speed  of  such  a  meter  is  a  measure  of  the  current  or  the  rate  of  flow  of  the 
electricity  through  the  motor  element  and  each  revolution  of  the  motor  corresponds  to  a 
given  quantity  of  electricity.  Then  by  connecting  a  revolution  counter  to  this  motor  gen- 
erator we  have  a  means  of  recording  the  total  quantity  of  electricity  in  ampere  hours  that 
is  passed  through  the  meter. 


CIRCUIT  BREAKER  WITH 
TRIPPING   MAGNET 


TO  LOAD 

Fig.     85 — Fundamental     Charging    Circuit    with    Overload    and    Underload    Circuit    Breaker    and    Ampere 

Hour  Meter. 


During  discharge  of  a  storage  battery  the  pointer  of  the  ampere  hour  meter 
moves  from  the  zero  position  of  the  scale  in  a  clockwise  direction  toward  the  full 
scale  reading,  but  when  the  battery  is  placed  on  charge  the  pointer  moves  in  a 
counter-clockwise  direction  toward  the  zero  division  of  the  scale.  When  it 
has  reached  this  point,  the  contacts  A  and  B  Fig.  85,  are  closed  and  the  circuit 
breaker  is  tripped  by  the  no  voltage  release  magnet  M.  This  cuts  off  the  charg- 
ing current.  Current  flows  through  winding  M  only  during  the  time  required 
to  throw  the  breaker,  the  current  being  turned  off  automatically,  through  a  set 
of  contacts,  by  the  breaker  armature.  The  circuit  for  the  no-voltage  release 
magnet  is  closed  when  the  circuit  breaker  proper  is  set  by  hand. 

Fig.  85  is  a  fundamental  diagram  of  a  charge  and  discharge  circuit  of  a  storage  battery 
installation  having  a  Sangamo  ampere  hour  meter.  The  meter  is  connected  in  the  circuit 
in  such  a  way  as  to  indicate  the  quantity  of  current  taken  in  by  the  battery  during  the 
charging  period  and  that  taken  out  by  the  motor  generator  or  auxiliary  lights  on  discharge. 


74 


PRACTICAL   WIRELESS   TELEGRAPHY. 


The  direction  of  the  current  through  the  ampere  hour  meter  on  discharge  is  opposite  to 
that  when  the  battery  is  on  ''charge,"  hence  fye  movement  of  the  motor  is  reversed  accord- 
ingly. 

For  sake  of  clearness  the  necessary  switches  for  placing  the  battery  on  charge  or  dis- 
charge have  been  omitted. 

The  short  arrows,  Fig.  85,  indicate  the  direction  of  the  charging  current  while  the  bat- 
teries are  on  charge  and  the  long  arrows  indicate  the  direction  of  current  during  discharge. 
This  diagram  is  not  intended  to  convey  a  description  of  the  mechanical  construction  or 
operation  of  the  circuit  breaker,  but  merely  serves  to  show  the  complete  fundamental  charg- 
ing circuit. 

73.  Overcharge. — Theoretically  we  should  be  enabled  to  draw  the  same 

quantity  of  current  from  a  storage 
battery  upon  discharge  as  put  into 
it  on  charge,  but  practically  this 
cannot  quite  be  done.  The  plates 
of  the  battery  must  be  given  an 
occasional  overcharge  to  keep  them 
in  good  working  condition,  that 
is,  there  must  be  put  into  the  bat- 
tery under  normal  conditions  a 
greater  quantity  of  current  than  is 
taken  out  of  it.  Ampere  hour 
meters  are  constructed  to  take  care 
of  the  required  overcharge  auto- 
matically by  a  device  known  as  a 
resistor  which  cannot  be  explained 
in  detail  here.  Briefly  it  causes  the 
pointer  of  the  ampere  hour  meter 
to  move  to  the  zero  position  of  the 
scale  from  a  given  higher  reading, 
at  a  slower  rate  than  that  which 
brought  it  to  the  maximum  reading 
thus  giving  the  requisite  overcharge.  But  with  all  this,  the  readings  of  the  ampere 
hour  meter  can  not  quite  keep  pace  with  the  state  of  the  storage  battery,  hence,  it 
is  necessary  from  time  to  time  to  move  the  pointer  of  the  ampere  hour  meter  from 
the  zero  position  to  a  reading  corresponding  to  from  20  to  50  ampere  hours, 
after  which  the  battery  is  placed  on  charge  until  the  pointer  again  returns  to  the 
zero  position. 

74.  How  to  Charge  a  Battery  when  the  Voltage  Exceeds  that  of  the  Charg- 
ing Generator. — As  mentioned  in  a  previous  paragraph,  the  voltage  of  the 
charging  generator  must  exceed  the  combined  fully  charged  voltage  of  the  cells. 
Since  the  auxiliary  battery  for  a  radio  set  consists  of  60  cells,  and  their  voltage 
combined  is  126  volts,  they  cannot  be  charged  in  series  from  a  110  volt  source  of 
direct  current.     The  battery  must  be  split  into  two  units  of  30  cells  each,  which 
are  connected  in  parallel  and  finally  to  the  terminals  of  a  charging  generator,  but 
on  discharge  the  cells  are  connected  in  series. 

A  completely  satisfactory  diagram  of  this  connection  appears  in  Fig.  86,  where  the  four- 
blade  switch,  when  thrown  to  the  "charge"  position,  connects  battery  A  and  battery  B  in 
parallel,  but  in  the  discharge  position  reconnects  them  in  series  giving  approximately  126 
volts  for  the  operation  of  a  standard  motor  generator.  This  diagram  shows  (1)  the  position 
of  the  charging  resistance  and  the  underload  circuit  breaker  in  the  charging  circuit,  (2) 
the  voltage  of  the  generator,  (3)  the  voltage  of  the  individual  battery  units  and  (4)  the 
final  discharge  voltage  of  the  cells.  The  charging  dynamo  in  the  case  of  a  marine  radio 
installation  is  the  ship's  generator,  which  is  situated  at  some  point  in  the  ship's  engine  room. 

It  has  been  mentioned  that  two  values  of  charging  current  are  employed  for  the  exide 
type  of  cells,  a  heavy  value  for  the  start  of  the  charge  and  a  lesser  value  at  the  completion 
of  the  charge,  but  no  special  appliances  are  fitted  to  the  Electric  Storage  Battery  Company's 


Fig.    85a— Portable    Exide    Storage    Cell,     Supplied    with 
Marconi  Emergency  Sets. 


STORAGE    BATTERIES   AND   CHARGING    CIRCUITS. 


75 


76 


PRACTICAL  WIRELESS  TELEGRAPHY. 


NE6AT!V£ 

PUTC 


ship's  charging  panel  whereby  the.se  two  values  can  be  obtained.  The  flow  of  charging 
current  is  automatically  taken  care  of  by  the  battery,  the  counter  E.  M.  F.  of  which  rises 
as  the  charge  progresses.  This  gradually  reduces  the  charging  current  without  extra 
appliances. 

75.  How  to  Determine  the  Polarity  of  the  Charging  Generator. — We  have 

mentioned  that  during  charge  the 
positive  pole  of  the  storage  bat- 
teries must  be  connected  to  the 
positive  pole  of  the  dynamo. 

The  polarity  of  the  charging 
mains  may  be  determined  in  three 
ways: 

(1)  By  a  direct  current  voltmeter  of 
the  magnetic  type; 

(2)  By    an    electrochemical   polarity 
indicator; 

(3)  By  dipping  the  terminals  of  the 
dynamo   in   a  glass   of  plain   or 
salt  water. 

Direct  current  voltmeters  with 
magnetic  mechanism  have  a  (-J-)  and 
( — )  mark  on  the  binding  posts.  If 
connected  improperly  to  a  source  of 
direct  current  the  pointer  instead  of 
swinging  in  the  direction  of  the  full 
scale  position  will  move  to  the  left  of 
the  zero  position,  but  when  connected 
properly,  the  pointer  moves  from  left 
to  right.  The  wire  of  the  dynamo 
Fig.  86a— Detail  of  Exide  11  M.  V.  Storage  Cell.  connected  to  the  (+)  binding  post 

of  the  voltmeter  is  the  positive  wire  and  the  other,  of  course,  the  negative  wire. 

Chemical  polarity  indicators  have  a  solution  of  iodide  of  potassium  mixed  with  starch 

sealed   in   a   glass   tube   provided   with   terminals.     When   connected   to   the   terminals   of   a 

charging   line,   current  flows    through  the   liquid   and   decomposes  the  solution,   turning  the 

positive  terminal  of  the  tube  blue. 

The  polarity  of  the  charging  source  may  be  ascertained  by  dipping  the  terminals  of  a 

D.  C.  line  in  a  glass  of  plain  water  as  in  Fig.  87  or 

preferably  salt   water.       Bubbles  will  appear  at  the  nega-      VWfflMfljfc  4" 

tive  terminal  of  the  line.     The  negative  wire  should  be 

connected    to    the    negative    pole    or    terminal    of    the 

battery. 

76.  Determination  of  the   State  of  Charge 
and   Discharge  of  a   Battery. — The   following 
instructions  apply  to  the  lead  plate  cell.     It  is 
not  difficult  to  determine  the  state  of  charge  and 
discharge  of  a  storage  battery  if  the  circuit  in- 
cludes an  amperehour  meter.     To  illustrate  the 
utility  of  this  instrument,  assume  a  60  ampere- 
hour  battery  which  is  being  discharged  through 
the   windings    of      a   motor   generator.       Fully 


BUBBLES 


charged,   the  pointer  of  the  amperehour  meter    Fig.   87— Method   of   Determining  the 
rests  at  zero  but  as  the  discharge,  progresses  the 

pointer  gradually  moves  in  a  clockwise  direction  and  if  it  then  rests  at  40,  it  is  an 
indication  that  40  amperehours  of  current  have  been  taken  from  the  battery 
which,  of  course,  is  not  completely  discharged.  In  order  that  the  battery  may  be 
ready  always  for  emergency  use,  the  changeover  switch  on  the  charging  panel 
should  be  immediately  thrown  to  the  "charge"  position,  the  underload  circuit 
breaker  closed,  whereupon  the  pointer  of  the  amperehour  meter  will,  as  the  charge 
progresses,  move  counter  clockwise  or  towards  the  zero  position  of  the  scale  which 


STORAGE    BATTERIES    AND   CHARGING    CIRCUITS.  77 

when  attained  throws  off  the  charging  current  automatically  by  means  of  the 
no  voltage  release  magnet  attached  to  the  underload  circuit  breaker.* 

In  event  the  equipment  is  not  fitted  with  an  amperehour  meter,  the  full 
charged  condition  of  the  chloride  cell  may  be  determined,  by  charging  at  normal 
rates  for  two  hours  after  gassing  begins  and  for  the  exide  cell,  one  hour  after 
gassing  begins. 

Each  cell  should  then  indicate  2.08  volts  on  open  circuit  and  the  gravity  of  the 
solution  should  have  reached  a  maximum  value. 

The  operator  should  understand  that  a  storage  cell  is  considered  to  be  fully 
charged  when  during  charging  there  is  no  further  rise  in  either  gravity  or  voltage ; 
and  that  in  any  case  a  maximum  value  of  voltage  and  gravity  should  be  aimed  for 
rather  than  a  final  value. 

The  state  of  a  storage  battery  may  be  obtained  in  another  way  by  placing  it 
on  discharge  at  normal  rates  and  observing  the  reading  of  the  voltmeter  with  the 
current  flowing.  If  the  reading  per  cell  is  less  than  1.8  volts,  the  battery  should 
be  placed  on  charge  immediately.  It  should  be  kept  in  mind  that  the  specific 
gravity  of  the  electrolyte  falls  on  discharge  and  rises  on  charge,  hence  a  hydro- 
meter may  be  used  in  place  of  the  amperehour  meter  for  determining  the  condi- 
tion of  a  cell  but  the  amperehour  meter  is  the  most  convenient  for  the  purpose. 

In  large  isolated  battery  units,  a  temperature  correction  table  is  given  to  permit  changes 
in  temperature  to  be  accounted  for  when  taking  readings  of  the  specific  gravity,  but  such 
tables  are  not  furnished  for  the  battery  sets  employed  in  marine  radio  sets. 

77.  Keeping  the  Level  of  the  Electrolyte. — There  is  no  loss  of  electrolyte 
in  the   charge  or  discharge  of  a  battery,  except  the  loss  due  to  spraying.     The  acid  does 
not  evaporate  but  the  water  does.     Hence  from  day  to  day  enough  water  must  be  added  to 
keep  the  electrolyte  at  a  level  of  ]/2  to  24  inches  above  the  plates. 

Replenishing  of  the  water  is  more  important  and  must  be  attended  more  frequently  in 
hot  climates  than  in  temperate  zones. 

78.  Protecting  the  Cells  from  Acid  Spray. — At  the  completion  of  a  charge, 
all  cells  should  be  thoroughly  wiped  off  first  with  a  damp  cloth  followed  by  a  dry  cloth. 
Surface  leakage  of  the  current  is  thus  prevented. 

The  cells  should  also  be  protected  from  salt  spray,  the  accumulation  of  dirt  and  dust, 
etc.  Contacts  and  connections  must  be  kept  scrupulously  clean  and  should  be  thoroughly 
examined  from  time  to  time. 

79.  General  Instructions  for  the  Portable  Chloride  type  of  Accumulators. 
—The  following  instructions  apply  to  the  portable  "chloride"  accumulator  of  the 

Electric  Storage  Battery  Company : 

The  proper  polarity  having  been  obtained,  charge  the  battery  at  the  rate  given  on  the 
name  plate  until  there  is  no  further  rise  in  the  voltage  of  each  cell  of  the  battery  and  each 
cell  has  been  gassing  or  bubbling  freely  for  at  least  two  hours  and  there  is  not  further  rise 
in  the  specific  gravity  of  the  electrolyte  over  the  same  period. 

If  the  temperature  of  the  electrolyte  rises  to  100  deg.  Fahrenheit  during  the  charge,  the 
current  should  be  reduced  or  stopped  until  it  lowers. 

The  voltage  at  the  end  of  the  charge  may  be  between  2.4  and  2.6  volts  per  cell,  depend- 
ing on  their  temperature  and  age. 

The  higher  voltages  are  obtained  on  new  batteries  with  low  temperature ;  on  all  bat- 
teries at  high  temperatures,  the  lower  voltages  are  obtained. 

It  therefore  must  be  understood  that  in  determining  the  completion  of  a  charge,  a  fixed 
or  definite  voltage  is  not  to  be  considered,  but  rather  a  maximum  voltage  as  indicated  by 
there  being  no  further  rise  in  the  voltage  over  a  period  of  two  hours.  It  is  of  the  utmost 
importance  that  the  charge  be  complete. 

Great  care  must  be  taken  not  to  bring  a  naked  flame  near  the  openings  in  the  top  of  the 
battery  during  or  immediately  following  a  charge. 

The  proper  specific  gravity  of  the  electrolyte  at  the  end  of  charge  is  1.210,  but  a  varia- 
tion of  from  1.205  to  1.215  is  allowable. 

*As  already  explained  the  battery  must  be  given  a  small  overcharge  occasionally  and  therefore 
when  the  pointer  has  returned  to  the  zero  position,  it  is  moved  by  hand  to  point  IS  or  20  (or  possibly  higher 
depending  upon  the  capacity  of  the  battery)  the  battery  being  again  placed  on  charge  until  the  pointer  returns 
to  the  zerp  po$ition. 


78  PRACTICAL   WIRELESS   TELEGRAPHY. 

Do  not  adjust  the  specific  gravity  except  when  the  battery  is  fully  charged. 

In  ordinary  use,  there  is  no  occasion  to  adjust  the  electrolyte  of  the  cell  oftener  than 
once  in  every  three  or  four  years. 

After  adjusting,  charge  for  an  hour  in  order  to  thoroughly  mix  the  liquid  just  added 
with  the  electrolyte. 

By  all  means  do  not  add  electrolyte  until  it  is  determined  that  the  specific  gravity  can- 
not be  brought  up  to  the  proper  point  by  charging. 

The  sediment  which  gradually  accumulates  in  the  bottom  of  the  jars  must  not  be 
allowed  to  touch  the  bottom  of  the  plates  as  if  it  does  serious  injury  may  result.  It  is 
best  to  overhaul  the  cells  and  remove  the  sediment  at  least  once  a  year. 

If  the  battery  is  not  to  be  used  for  a  considerable  period,  care  should  be  taken  that  it 
is  charged  once  every  two  months  and  that  the  plates  are  kept  covered  by  regularly  adding 
water. 

If  it  is  not  possible  to  charge  at  least  once  every  two  months,  the  battery  may  be  taken 
entirely  out  of  service  as  follows :  Fully  charge  the  battery,  remove  the  vent  caps,  pour 
out  the  electrolyte  by  turning  the  case  upside  down,  fill  with  fresh,  pure  water  and  allow 
to  stand  12  or  15  hours.  Then  pour  off  the  water,  wipe  the  case  dry,  grease  the  terminals 
and  the  battery  can  stand  indefinitely. 

80.  General  Operating  Instructions  for  the  Exide  Cell.  The  following 
instructions  apply  specifically  to  the  Exide  cells  supplied  to  marine  radio  sets, 
particularly  the  60  cell  M.  V.  11  type.  (E.  S.  B.  Co.) 

Keep  the  level  of  the  electrolyte  always  above  the  top  of  the  plates  by  replacing  evapora- 
tion with  pure,  fresh  water  (never  anything  else)  to  a  height  of  one-half  inch  (not  more) 
above  top  of  plates.  The  best  time  for  adding  water  is  just  before  a  charge.  Do  not  use 
metallic  receptacles  for  holding  the  water. 

Keep  flames  of  all  kinds  (match,  candle,  lantern,  cigar,  etc.)  away  from  battery  at  all 
times. 

Battery  is  to  be  "floated"  at  all  times,  except  when  charging  or  discharging.  When 
floating,  both  lamps  on  battery  switchboard  will  burn  dimly.  If  either  lamp  goes  out, 
immediately  replace  it  with  another  of  same  rating. 

Twice  each  month,  preferably  when  in  port,  charge  the  battery  by  opening  the  6  pole 
switch,  closing  the  circuit  breaker,  and  again  closing  the  6  pole  switch  to  the  charge  side. 
Move  the  needle  of  the  ampere  hour  meter  back  to  about  50  and  charge  until  the  pilot  cell 
gravity  and  the  voltage  of  each  side  have  remained  constant  for  one  hour  and  all  cells 
have  been  gassing  or  bubbling  freely  for  the  same  length  of  time.  This  means  that,  under 
normal  floating  conditions,  the  charge  will  be  of  about  one  and  one-quarter  hours'  dura- 
tion. After  the  charge  reset  the  needle  of  the  ampere  hour  meter  to  zero.  Raise  the  cover 
of  the  battery  box  during  this  charge. 

After  a  discharge  of  any  kind  immediately  put  the  battery  on  charge  and  continue  the 
charging  until  the  needle  of  the  ampere  hour  meter  has  returned  to  zero. 

In  order  to  check  the  generator  polarity  and  to  guard  against  the  battery  becoming 
accidentally  discharged  through  the  reversal  of  the  generator,  read  the  voltmeter  irequent^. 
with  the  voltmeter  plug  in  openings  marked  "Bus." 

If  the  polarity  has  changed,  throw  over  the  switch  marked  "Reversing  Switch." 

Before  the  semi-monthly  charge,  read  and  record  the  specific  gravity  of  each  cell  of  the 
battery. 

On  the  day  of  the  semi-monthly  charge,  after  the  charge  has  been  completed,  read  and 
record  the  specific  gravity  of  the  pilot  cells. 

On  other  days  read  and  record  the  specific  gravity  of  the  pilot  cells  at  the  same  time 
each  day.  These  readings  will  indicate  the  state  of  charge  of  the  battery  and  will  be  a 
check  on  the  "Floating." 

On  the  semi-monthly  charges  the  vent  plugs  should  be  replaced  after  charges  are  com- 
pleted. 

If  the  gravity  of  any  cell  shows  a  marked  falling  off  relative  to  the  rest  of  the  cells, 
promptly  investigate  the  cause  and  correct  it. 

If  a  jar  develops  a  leak,  promptly  replace  it. 

If  a  cell  becomes  "dead"  from  a  leaky  jar,  cut  it  out  of  circuit  by  opening  up  the  con- 
nector and  restore  the  circuit  with  a  jumper. 

When  charging,  keep  the  bus  voltage  at  110  volts,  as  if  it  is  low  the  charging  rate  will 
be  reduced  and  the  time  required  to  charge  correspondingly  increased. 

Keep  everything  about  the  battery  clean  and  dry. 


STORAGE   BATTERIES   AND   CHARGING   CIRCUITS.  79 

Keep  terminals  and  connections  tight  and  free  from  corrosion. 
Do  not  allow  any  impurities  to  get  into  a  cell. 

81.  The  Edison  Storage  Battery. — The   Edison  cell  differs  from  the  lead 
cell  both  in  the  construction  of  the  plates  and  the  electrolyte.     The  active  mate- 
rials of  this  cell  are  iron  oxide  and  nickel  hydrate.    The  electrolyte  is  a  21  per 
cent,    solution   of   potassium   hydrate   mixed   with   a   small    amount   of   lithium 
hydrate. 

The  negative  plate  consists  of  a  nickel-plated  steel  grid,  into  the  pockets  of  which  are 
hydraulically  pressed,  perforated,  corrugated  steel  pockets  which  have  been  filled  and 
packed  with  iron  oxide,  to  which  has  been  added  a  small  amount  of  metallic  mercury. 

The  positive  plate  consists  of  a  nickel-steel  grid  to  which  are  secured  perforated  steel 
tubes  reinforced  by  seamless  steel  springs.  These  tubes  are  filled  with  alternate  layers  of 
nickel  hydrate  and  very  thin  plate  nickel  firmly  and  carefully  packed  by  a  loading  machine. 

82.  The  Charge  and  Discharge  of  the  Edison  Cell. — The  process  taking 
place  in  an  Edison  cell  during  charge  and  discharge  is  as  follows : 

The  first  charging  of  an  Edison  cell  reduces  the  iron  oxide  to  a  metallic  iron,  while 
converting  the  nickel  hydrate  to  a  very  high  oxide,  black  in  color.  On  discharge,  the 
metallic  iron  goes  back  to  iron  oxide  and  the  high  nickel  oxide  goes  to  a  lower  oxide,  but 
not  to  its  original  form  of  green  hydrate.  On  every  cycle  thereafter  the  negative  charges 
to  metallic  iron  and  discharges  to  iron  oxide  while  the  positive  plate  charges  to  a  high 


Fig.     87a. — Group     of    Electric     Storage     Battery     Co.'s     Portable 
Chloride     Storage     Cells. 

nickel  oxide.  Current  passing  in  the  direction  of  charge  or  discharge,  decomposes  the 
potassium  hydrate  of  the  electrolyte  and  the  oxidation  and  reduction  of  the  electrode  are 
brought  about  by  the  action  of  its  elements.  An  amount  of  potassium  hydrate  equal  to 
that  decomposed  is  always  reformed  at  one  of  the  electrodes  by  a  secondary  chemical 
reaction,'  in  consequence  there  is  none  of  it  lost  and  its  density  remains  constant. 

The  final  result  of  charging  is,  therefore,  the  transference  of  oxygen  from  the  iron  to 
the  nickel  electrodes  and  that  of  discharging  is  the  transference  back  again. 

A  hydrometer  reading  of  this  cell  is  not  required  as  the  specific  gravity  of  the  elec- 
trolyte does  not  change  with  the  state  of  charge  or  discharge  and  consequently  the  only 
direct  method  to  measure  the  state  of  charge  is  with  an  ampere  hour  meter,  the  hydrometer 
being  of  no  use. 


PART  VI. 


THE  RADIO  TRANSMITTER 


CONDENSERS— OSCILLATION       GENERATORS— RADIATION 
ELECTRICAL     WAVES— DAMPING   OF   OSCILLATIONS. 


OF 


83.  METHODS    OF   GENERATING   RADIO   FREQUENCY    CURRENT. 

84.  THE     CONDENSER.     85.  CONNECTIONS     FOR     CONDENSERS. 
86.  How  TO  PLACE  A  CHARGE  IN  A  CONDENSER.     87.  ANALYSIS 
OF  A  SPARK  DISCHARGE.     88.  EFFECT  OF  RESISTANCE  ON  OSCIL- 
LATIONS.    89.  ELECTRICAL  RESONANCE.     90.  THE  OPEN   CIR- 
CUIT OSCILLATOR.     91.  THE  LENGTH  OF  THE  ELECTRIC  WAVE. 
92.  THE  DETERMINATION  OF  WAVE  LENGTH  FROM  THE  INDUC- 
TANCE AND  CAPACITY.     93.  LOGARITHMIC  DECREMENT  OF  THE 
OSCILLATIONS.     94.  METHODS  OF  EXCITING  OSCILLATIONS  IN 
AN  AERIAL.     95.  THE  REACTION   OF  COUPLED  CIRCUITS.     96. 
THE  STANDARD  WAVES  OF  COMMERCIAL  WIRELESS  TELEGRAPHY. 
97.  FUNDAMENTAL   CIRCUIT   OF   A   COMPLETE   RADIO   TRANS- 
MITTER.    98.  SIMPLE    EXPLANATION    OF    THE    CIRCUITS.     99. 
NUMERICAL  VALUES  FOR  A  STANDARD  RADIO  SET. 


Fig.   88 — Oscillations  of  Constant  Amplitude. 
CONSTANT    AMPLITUDE 


The  electric  waves  for  commercial  wireless  telegraphy  are  set  into  motion  by 
alternating"  currents  or  electrical  oscillations  at  frequencies  varying  from  25,000 
to  1,000,000  cycles  per  second.  We  shall  confine  this  chapter  to  an  explanation 
of  the  apparatus  for  the  production  of  damped  electrical  oscillations  which  are 
employed  almost  universally  for  wireless  ship  to  shore  communication. 

83.  Methods  of  Generating  Radio  Frequency  Current. — In  order  to  distin- 
guish alternating  currents  of  the  order  of  frequency  employed  in  wireless  teleg- 
raphy from  those  of.  a  lower  frequency  corresponding  to  audible  vibrations, 

the  following  arbitrary  terms  are  in 
use;  an  alternating  current  of  fre- 
quency in  excess  of  10,000  cycles  per 
second,  is  termed  a  current  of  radio- 
frequency;  below  10,000  cycles  per 
second,  it  is  called  a  current  of 
audio-frequency. 

A  current  of  radio-frequency 
may  consist  of  either  continuous  or 
discontinuous  oscillations.  A  group 
of  oscillations  of  continuous  ampli- 
tude are  shown  in  Fig.  88;  a  group 
of  discontinuous  oscillations  appear 
in  Fig.  89. 

Continuous  oscillations  are  gen- 
Fig.  89— Oscillations  of  Decreasing  Amplitude.  Crated  : 


AMPLITUDE 


THE   RADIO   TRANSMITTER. 


81 


(1)  By  the  radio  frequency  alternator; 

(2)  By  some  form  of  the  D.  C.  arc  generator; 

(3)  By  a  battery  of  vacuum  tube  bulb. 
Discontinuous  oscillations  are  generated : 

(1)  By  the  charge  and  discharge  of  a  Leyden  jar  or  battery  of  condensers. 

Electric  waves  set  into  motion  by  alternating-  current  of  constant  amplitude 
are  called  continuous  or  undamped  waves  and  those  set  into  motion  by  discon- 
tinuous oscillations  occurring  in  groups  are  called  discontinuous  waves.  Electrical 
oscillations  of  decaying  amplitude  are  also  called  damped  oscillations. 

Since  damped  oscillations  are  generated  by  the  periodic  charge  and  discharge 
of  some  form  of  condenser,  certain  fundamental  facts  concerning  the  condenser 
will  now  be  considered. 

84.  The  Condenser. — The  condenser  may  be  defined  as  a  device  to  store  up 
electrical  energy  in  electrostatic  form  and  the  ability  of  any  material  to  do  this,  is 
termed  its  electrostatic  capacity  or  simply  capacity. 

The  most  common  form  of  condenser  used  in  wireless 
systems  is  the  Leyden  jar  shown  in  Fig.  90.  A  glass  jar  having 
walls  about  one-eighth  of  an  inch  in  thickness  is  coated  with 
tin-foil  to  within  two  inches  of  the  top,  both  inside  and  outside, 
connection  being  made  to  the  inner  coating  by  means  of  a  chain 
or  copper  strip.  Modern  jars  are  coated  with  a  deposit  of 
silver  or  copper  by  an  electrolytic  process.  Another  type  of 
condenser  consists  of  a  flat  plate  of  glass  coated  with  tinfoil 
or  copper  immersed  in  oil. 

Condensers  for  radio  telegraphy  may  be  generally  classified 
under  two  headings: 

(1)  High  Potential  Condensers; 

(2)  Low  Potential  Condensers. 

Those  in  the  first  category  have  an  insulating  medium  be- 
tween opposed  conducting  surfaces  of  glass,  micanite,  ebony,  or 
the  plates  may  be  sealed  in  an  air-tight  chamber  pumped  to  a 
pressure  of  250  pounds. 

Those  in  the  second  category  have  an  insulating  medium  of 
paper,  thin  sheets  of  hard  rubber,  or  specially  impregnated 
cloth. 

High  potential  condensers  are  employed  in  circuits,  where 
the  pressure  is  several  thousand  volts,  while  low  potential  con- 
densers are  generally  used   in   circuits  having  less  than   500 
volts,  the  latter  figure  being  an  arbitrary  one. 
A  number  of  condenser  jars  or  Leyden  jars  connected  together  are  termed  a  battery  of 
condensers. 

85.  Connections  for  Condensers. — Condensers  may  be  connected  either  in 
series  or  in  parallel.    If  connected  in 

parallel  as  in  Fig.  91  the  total  capacity     ~ 
is  determined  by  adding  together  the 
individual  values,  or: 

C  =  C-l  +  C-2  +  C-3. 

Thus  the  jars  in  Fig.  91  have 
combined  capacity  of  .002  -j-  .003  + 
.004  or  .009  microfarad. 

A  series  connection  is  shown  in 
the  diagram,  Fig.  92,  where  the  outer 
coating  of  one  jar  is  connected  to  the 

outer  coating  of  the  next,  the  inner     

coating  of  this  jar  connected  to  the 
inner  coating  of  the  next  jar  and  so  on. 

If  the  jars  have  equal  values  of  capacity,  their  combined  capacity  is  obtained 


Fig.  90 — Simple  Copper  Plated 
Leyden  Jar. 


Fig.  91 — Condenser  Jars  in  Parallel. 


82 


PRACTICAL   WIRELESS   TELEGRAPHY. 


by  merely  dividing  the  capacity  of  one  jar  by  the  number  of  jars  in  the  cir- 
cuit,  but  if  the  jars  have  unequal  values,  the  formula  of  reciprocals  applies: 


C  = 

1 


.ooz 

MFD. 


1  1 

C-l     C-2     C-3 

Therefore  the  capacity  of  the  jars 
1 


of  Fig.  92  = 


1 


1 


Fig.  92 — Condenser  Jars  in  Series. 


.004 


.002       .003 
:    .00092  microfarads. 

86.  How  to  Place  a  Charge  in  a  Condenser. — The  condenser  of  a  radio 
transmitter  may  be  charged  from  two  sources: 

(1)  By  the  step-up  voltage  induction  coil; 

(2)  By  the  alternating  current  step-up  transformer. 

The  alternating  current  transformer  is  almost  universally  employed  in  radio  sets  t® 
charge  the  condenser,  because  it  overcomes  the  limitations  of  the  magnetic  interrupter. 
A  transformer  can  convert  a  practically  unlimited  amount  of  power  into  high  voltage 
current  whereas  the  induction  coil  with  a  magnetic  interrupter  is  limited  to  about  1  K.  W. 

A  condenser  can  be  given  a  heavy  charge  by  the  apparatus  shown  in  Fig.  93.  A  high- 
voltage  A.  C.  transformer,  10,000  to  30,000  volts,  is  connected  to  the  terminals  of  three 
condenser  jars,  and  when  the  current  is  turned  on,  a  violent  discharge  takes  place  at  the 
gap  S-l. 


,- 

10  VOLT       * 
AC.           « 

i 

_ 

T 

|                  { 

± 
—  t- 
=c 

>      li 

gj 

Fig.  93 — Circuit  for  Production  of  Radio  Frequency  Oscillations. 


In  the  diagram  Fig.  93  P  is  the  primary  winding  of  an  alternating  current  transformer 
taking  in  current  at  the  pressure  of  110  volts,  which  is  transformed  in  the  secondary  wind- 
ing S  to  a  pressure  of  20,000  volts.  C  is  a  battery  of  condensers  and  L  a  radio-frequency  coil 
consisting  of  a  few  turns  of  rather  coarse  wire  or  copper  tubing  (without  an  iron  core). 
S-l  is  the  spark  discharge  gap,  the  usual  form  being  two  brass  or  zinc  rods  about  ]/2  inch 
or  less  in  diameter. 

When  current  is  flowing  through  the  primary  winding  P,  with  correct  separation  of  the 
spark  gap  electrodes,  a  violent  spark  discharge  will  take  place  following  each  alternation 


THE   RADIO   TRANSMITTER. 


83 


of  the  charging  current  or  at  tfie  points,  A,  B,  C,  D,  E  and  F  shown.  The  discharge  upon 
analysis  is  found  to  consist  of  groups  of  radio-frequent  oscillations  which  alternate  through 
the  condenser  and  inductance  at  an  extremely  rapid  rate  per  second  of  time.  More  clearly, 
if  the  frequency  of  the  charging  current  is  500  cycles  per  second  (or  1,000  alternations),  the 
condenser  C  will  be  charged  1,000  times  per  second  and  1,000  spark  discharges  will  take  place 
at  the  gap  S.  This  will  result  in  the  flow  of  1,000  groups  of  radio-frequent  oscillations 
through  the  discharge  circuit. 


Fig.   94 — Complete   Cycle   of  Events  in  the   Discharge   of  a   Condenser. 

The  term  spark  frequency  is  employed  to  designate  the  number  of  spark  dis- 
charges bridging  the  gap  per  second  of  time.  In  certain  types  of  radio  sets  the 
spark  frequency  equals  the  alternations  of  the  charging  current,  but  in  other 
types  the  frequency  of  the  sparks  may  vary  in  accordance  with  the  general  design 
of  the  apparatus  (to  be  explained  in  the  chapter  following). 

87.  Analysis  of  a  Spark  Discharge. — A  spark  discharge  such  as  that 
created  by  the  condenser  Fig.  94  consists  of  a  series  of  cycles  of  alternating  cur- 
rent of  constantly  decreasing  amplitude,  the  frequency  of  which  may  be  very 
great.  These  are  known  as  damped  oscillations,  a  single  group  of  which  are 
shown  in  Fig.  89. 

The  complete  cycle  of  events  in  the  discharge  of  a  condenser  may  be  summarized  as 
follows:  Referring  to  the  diagrams,  Fig.  94-a-b-c-d-e :  Just  previous  to  the  first  discharge 
the  charge  in  the  condenser  takes  the  form  of  an  electrostatic  field  which  is  stored  up  between 
the  plates  as  in  Fig.  94a.  The  strain  is  such  that  a  spark  leaps  from  the  positive  electrode  to 
the  negative  electrode  of  the  gap,  that  is  the  collapse  of  the  electrostatic  field  causes  a  current 
to  flow  across  the  spark  gap  through  the  inductance  L  which  sets  up  a  magnetic  field  around 
the  coil  L  and  connecting  leads  as  in  Fig.  94b.  Since  all  of  the  electrostatic  field  is  converted 
to  current,  the  magnetic  lines  of  force  collapse  back  upon  the  coil  L  (and  connecting  leads), 
the  induced  current  resulting  therefrom  recharging  the  condenser  to  the  opposite  polarity  as 
in  Fig.  94c.  Although  charged  in  the  opposite  sense,  a  quantity  of  electricity  less  than 
that  of  the  initial  charge  is  stored  between  the  plates,  some  of  the  energy  having  been  expended 
by  the  resistance  of  the  circuit  and  the  production  of  heat,  light  and  sound.  The  condenser 
now  discharges  across  the  spark  gap  in  the  direction  opposite  to  that  indicated  in  94b  and  the 
coil  L  once  more  is  surrounded  by  a  field  (Fig.  94d),  but  which  is  opposite  in  polarity  to  that 
of  Fig.  94b.  These  lines  of  force  collapse  and  recharge  the  condenser  to  the  polarity  indicated 
at  the  beginning  of  the  cycle  in  Fig.  94a,  but  a  charge  less  in  quantity  is  stored  between  its 
plates  than  at  the  beginning  of  discharge. 

All  this  may  be  summed  up  by  saying  that  when  an  isolated  charge  of  elec- 
tricity is  applied  to  a  condenser,  and  the  plates  are  connected  together  by  an 
external  circuit,  their  charges  do  not  completely  neutralize  at  the  first  instant  of 
discharge  but,  in  fact,  several  alterations  of  current  take  place  before  equilibrium 
is  restored. 

This  gradual  extraction  of  energy  from  the  oscillations  set  up  by  the  condenser  is  termed 
the  damping  of  the  oscillations  and  the  decrease  in  amplitude  of  the  successive  cycles  can  be 
expressed  in  a  logarithmic  percentage. 

In  the  oscillation  circuits  of  a  commercial  radio-transmitter  from  a  number  of  complete) 


Footnote:     The  student  should  compare  the  time  period  of  a  single  cycle  in  a  condenser  discharge  to  that 

of  a  500  cycle  alternator.  A  complete  cycle  takes  place  in  the  circuit  of  the  latter  in of  a  second  but  a 

v  500 

single  cycle   in   a  group  of  oscillations  of  a  condenser  discharge   may  take   place  in  various  fractions   of  a 

second  varying  from  —    of  a  second  to  • of  a  second. 


25,000 


2,000,000 


84  PRACTICAL   WIRELESS   TELEGRAPHY. 

cycles  of  current  may  take  place,  before  the  charge  originally  placed  in  the  condenser  is  com- 
pletely dissipated. 

The  frequency  of  the  oscillations  in  a  condenser  discharge  circuit  may  be  verv  great,  in 

1 
fact,  a  single  cycle  may  take  place  in  —          —  of  a  second,  or,  in  other  words,  at  the  rate  of 

1,000,000 
1,000,000  cycles  per  second,  or  at  even  higher  frequencies. 

The  frequency  of  the  oscillations  during  the  discharge  of  a  condenser  varies  inversely  as 
the  square  root  of  the  product  obtained  in  multiplying  the  inductance  of  the  circuit  in  henries 
by  the  capacity  in  farads,  or 

N-  1 

-2»VLC 

If  L  be  converted  to  centimeters  and  C  to  microfarads,  the  formula  becomes: 

5,033,000 

N  =i 


VLC 

As  an  example,  assume  that  the  condenser  C,  Fig.  95,  has  capacity  of  .001  microfarads  and 
the  coil  L  inductance  of  25,000  centimeters  (or  25  microhenries)  then  the  frequency 

5,033,000 
N  :=  —  -  n=  1,000,000  cycles  per  second  approximately. 


The  quotient  of  this  problem  does  not  imply  that  1,000,000  cycles  of  current  actually  took 
place  in  one  second  but  that  during  the  period  of  oscillation,  the  condenser  discharged  through 
the  inductance  at  this  rate.  It  must  be  remembered  that  the  condenser  is  being  charged  by  an 
alternating  current  and  in  the  case  of  the  500  cycle  alternator  it  will  be  charged  1,000  times; 
hence  in  one  second  there  are  a  number  of  idle  periods  during  which  the  condenser  receives  no 
charge. 

The  student  should  not  confuse  the  actual  number  of  oscillations  in  a  circuit  such  as  shown 
in  Fig.  95  with  the  frequency  of  the  oscillations.  As  has  just  been  explained  the  oscillation 

frequency  is  determined  by  the  dimensions 


(000 


—      24000  CID5.        (inductance  and  capacity)  of  the  oscillatory 

^-^-^— ^— ^  circuit  whereas  the  number  of  oscillations 

per  single  spark  is  a  function  of  the  total 
damping  of  the  circuit.  It  should  be  under- 
stood that  an  oscillation  circuit  like  that  of 
Fig.  95  is  charged  by  an  alternating  current 
always  will  produce,  during  discharge, 
oscillations  of  constantly  decreasing  ampli- 
.001  MFD  /fi\~  tude  or  damped  oscillations. 

A  circuit  like  that  of  Fig.  95  is 
termed  a  circuit  of  radio-frequency. 
The  complete  apparatus  may  be  called 


an  oscillation  generator  and  the  suc- 

Fig.  95 — Closed  Oscillation  Circuit.  ^  .. 

cessive  cycles  of  current  are  termed 

high  frequency  or  radio-frequency  electrical  oscillations.  When  this  apparatus 
is  employed  to  generate  oscillations  of  radio-frequency  for  the  production  of 
electric  waves,  the  circuit  is  denoted  as  the  closed  oscillatory  circuit  to  distinguish 
it  from  the  open  or  radiating  circuit. 

88.  Effect  of  Resistance  on  Oscillations. — If  in  any  given  oscillation  circuit, 


Footnote:     If  the  effective  resistance  of  R  of  an  oscillatory  circuit  be  taken  into  account,  the  fo/mula  for 

i    nr    R* 

frequency  becomes  N= \[ but  in  the  closed  oscillation  circuits  of  a  practical  radio  telegraph 

transmitter,   the  value  of  R  is   rather  small  and   is  usually   ignored. 


THE    RADIO    TRANSMITTER.  85 

a  certain  critical  value  of  resistance  is  exceeded,  the  discharge  of  the  condenser  unll  not  be 
oscillatory.    The  relation  between  the  resistance  of  a  circuit  and  the  non-oscillatory  condition 

is  expressed  as  follows :  If  the  resistance  R  is  greater  than  2  \l  -—  ,  the  circuit  is  non-oscilla- 
tory; if  resistance  is  equal  to  2    V — the  circuit  is  just  oscillatory.     If  the  resistance  is  less 

than  2  y  p—  the  circuit  will  he  oscillatory  and  will  be  suitable  for  the  production  of  radio- 
frequency  oscillation. 

In  this  equation  R  is  expressed  in  ohms,  L  in  henries,  and  C  in  farads.  The  value  of 
1  \/  —  is  seen  to  be  the  critical  resistance  to  cause  a  given  circuit  to  be  non-oscillatory. 

The  elements  of  a  closed  oscillation  circuit  in  a  radio-transmitter  are  connected  together 
with  very  heavy  stranded  copper  wire  or  copper  tubing  which,  excluding  the  resistance  of  the 
spark  gap  and  other  losses,  tends  to  keep  the  resistance  at  a  minimum ;  in  fact,  practical  oscil- 
lation circuits  do  not  have  anywhere  near  the  critical  value  of  resistance. 


Fig.   96 — Circuit  for  Demonstrating  the  Phenomenon   of  Resonance. 

Assuming  tV  resistance  of  the  oscillation  circuit  of  Fig.  95  to  be  negligible,  the  frequency 
of  the  oscillations  can  be  varied  over  given  practical  limits  by  variation  of  either  the  capacity 
or  the  inductance.  If  turns  are  added  at  the  coil  L,  the  current  oscillates  through  the  circuit 
at  a  lower  frequency ;  but  if  turns  are  taken  out  at  L,  the  frequency  of  the  oscillations  in- 
creases. In  a  similar  manner,  increase  of  capacity  at  the  condenser  C  will  reduce  the  fre- 
quency of  the  oscillations  and  decrease  of  capacity  will  increase  the  frequency  of  the  oscilla- 
tions. In  a  radio  transmitter  the  capacity  of  the  condenser  C  is  a  fixed  quantity,  and  there- 
fore, the  frequency  of  the  oscillations  is  changed  by  the  variable  inductance. 

89.  Electrical  Resonance. — The  phenomenon  of  resonance  is  very  striking 
in  circuits  of  radio-frequency.  In  order  that  the  energy  of  the  oscillations  flow- 
ing in  one  circuit  may  be  transferred  most  effectively  by  electromagnetic  induc- 
tion, the  two  circuits  must  have  the  same  natural  frequency  of  oscillation.  In 
the  diagram  of  Fig.  96  two  circuits  of  radio-frequency,  L,  C,  S  and  L1,  C1,  A  are 
magnetically  coupled  at  the  coils  L  and  L1.  If  circuit  L,  C,  S  is  set  into  oscilla- 
tion and  the  inductance  of  L1,  C1,  A  carefully  adjusted,  a  point  will  be  found 
where  the  hot  wire  ammeter  gives  a  maximum  deflection.  At  all  other  points  on 
the  inductance  L,  the  hot  wire  ammeter  will  show  a  lower  reading.  It  can  be 
shown  that  the  adjustment  of  the  second  circuit  where  the  deflection  of  the 
ammeter  is  maximum,  is  such  that  the  two  circuits  have  substantially  the  same 
period  of  oscillation  or  the  same  natural  frequency,  and  it  is  under  such  adjust- 
ments only  that  large  values  of  current  can  be  induced  in  the  circuit  L1,  C1,  A. 


86 


PRACTICAL  WIRELESS  TELEGRAPHY. 


It  was  shown  in  paragraphs  45  and  46  that  a  much  greater  current  will  flow  in  an  alternat- 
ing current  circuit  if  it  contains  inductance  and  capacity  of  such  value  that  the  reactance  of 
capacity  and  the  reactance  of  inductance  are  equal  and  opposite.  This  condition  exists  in 
the  circuit  L1,  C,  A  of  Fig.  96  when  the  ammeter  gives  the  maximum  deflection,  e.  g.,  the 
reactance  of  the  inductance  L1  equals  the  reactance  of  the  condenser  C1  and  therefore  the  flow 
of  current  in  circuit  L1,  C1,  is  governed  by  its  frictional  resistance.  The  maximum  amount  of 
energy  will  be  obtained  under  these  conditions. 

Two  or  more  circuits  of  radio-frequency  are  said  to  be  in  electrical  resonance 
when  the  product  of  the  inductance  multiplied  by  the  capacity  gives  a  like  figure 
in  each.  Or,  stated  in  another  way,  two  circuits  are  in  resonance  when  they  have 
the  same  oscillation  constant.  This  factor  is  obtained  from  the  VL  C.  Circuits 
having  like  oscillation  constants  will  have  the  same  discharge  frequency  when 
set  into  oscillation,  and,  therefore,  they  are  in  electrical  resonance.  Applying  this 


._j 


Fig.  97a  —  Plain  Aerial  Excitation. 

Fig.    97b  —  Showing   Position    of    Short  Wave   Condenser   and   Aerial   Tuning   Inductance. 
Fig.   97c  —  Illustrating  the  Effective  Capacity  of  an  Aerial. 

to  the  circuit  of  Fig.  96  we  see  that  no  matter  how  large  or  how  small  the  values 
of  L  and  C,  L1  and  C1  may  be,  if  V  L  C  =  V  L1  C1,  the  circuits  are  in  sub- 
stantial resonance. 

As  an  example:     If  in  Fig.  96,  L  =  25,000  centimeters;  C  =  .001  micro- 
farads; L1  =  5,000  centimeters  and  C1  =  .005  microfarads;  then  L   X   C  - 
L1  X  C1  (for  VL  C  or  VL1  C1  =  5). 


90.  The  Open  Circuit  Oscillator.  —  In  the  closed  oscillation  circuits  of  figures 
95  and  96,  the  capacity  of  the  condenser  and  the  inductance  of  the  coil  are  said 
to  be  concentrated,  the  static  field  being  stored  up  mainly  between  the  plates  of 
the  condenser  and  the  magnetic  field  mainly  about  the  coil. 

An  open  circuit  oscillator*,  such  as  shown  in  Fig.  97a,  ma*y  have  distributed 
inductance  and  capacity  or  both  distributed  and  concentrated  values  of  inductance 
and  capacity  as  in  Fig.  97b. 


*The    capacity    of    this   vertical    wire    lies    in    its    ability    to    store   up    static   lines    of   force   and    the 
inductance  lies  in  its  ability  to  store  up  magnetic  lines  of  force. 


THE   RADIO   TRANSMITTER. 


87 


In  these  diagrams  a  vertical  wire*  100  ft.  or  more  in  length  extends  into 
space  and  is  connected  to  earth  at  the  opposite  end.  A  spark  gap  is  included  in 
the  circuit  at  S-l.  Although  the  inductance  and  capacity  of  this  circuit  are  dis- 
tributed throughout  its  length,  it  remains  a  circuit  of  radio-frequency  capable 
of  being  set  into  oscillation.  If  the  spark  gap  S-l,  Fig.  97a,  is  connected  to  a 
source  of  high  voltage  S  such  as  the  secondary  winding  of  a  transformer,  an 
electrostatic  field  will  be  stored  up  in  the  region  about  the  wire  and  a  discharge 
will  take  place  across  the  spark  gap.  This  discharge  will  consist  of  radio- 
frequency  oscillations  like  those  of  closed  oscillation  circuits  with  the  exception 
that  a  certain  amount  of  the  energy  will  be  radiated  in  the  form  of  electro- 
magnetic waves. 

As  in  the  closed  circuit,  the  fre- 
quency of  the  antenna  oscillations  can 
be  increased  or  decreased  by  change 

of  inductance  or  capacity,  but  since  the  

vertical  conductor  has  fixed  dimen- 
sions, the  oscillation  frequency  in  prac- 
tice is  changed  by  artificial  means  at 
the  base  of  the  aerial.  For  example, 
in  the  apparatus  of  Fig.  97b,  the  fre- 
quency of  the  oscillations  can  be  re- 
duced by  increase  of  inductance  at  L 
or  it  may  be  increased  by  inserting  a 
condenser  at  C. 

In  the  diagram  of  Fig.  97c,  the 
effective  capacity  of  an  antenna  is 
represented  by  a  condenser  in  dotted 
lines.  This  in  no  measure  illustrates 


\ 


/                             ^-  ^—  —  -                        W-.  -—  —  •* 

\ 

/                /"               ^vlX                   * 

^             \ 

/         x           -T- 

\            \ 

/      X       —  .       ^ 

V           \ 

/       /               T            N 

\           \ 

\        \          \ 

\ii     s*'  ~~jt~  ^> 

\       \    •   \ 

/     ,    /     /       •      \ 

/        I      I       /              A              \ 

\       \       \ 

\       \         \ 
\       \        \ 

Fig.    98 — Electrostatic   Field   About   Aerial   Previous   t< 
Spark   Discharge. 


the  true  conditions  of  affairs  in  the  region  about  a  vertical  wire,  but  it  may  serve 
to  make  the  oscillating  properties  of  a  vertical  conductor  more  or  less  self-evident. 
An  open  circuit  oscillator  in  radio-telegraphy  is  known  as  an  aerial  or  an 
antenna,  and  it  is  said  to  be  a  radiator  of  electromagnetic  waves.  In  commercial 
practice,  aerials  may  have  2,  4  or  6  wires  connected  in  parallel. 

/n  r\ 


" 

LENGTH  OF 

-        +                         +•»•       — 

R                                                       A 

_  LENGTH  OF 

+  "t" 

Fig.   99 — Detached  Loops  of  Electrostatic   Strain. 

The  complete  process  of  radiation  of  electromagnetic  waves  is  a  complex  subject  which 
will  only  be  treated  in  a  popular  manner  here.  The  vertical  wire  of  Fig.  98  and  the  earth 
may  be  considered  as  two  sides  of  a  condenser  having  a  certain  capacity,  and  when  the  spark 
gap  is  connecled  to  a  source  of  high  voltage,  electrostatic  lines  of  force  will  be  stored  up  ii> 
the  space  surrounding  the  aerial  and  when  the  maximum  charge  has  been  reached,  a  discharge 
takes  place  across  the  spark  gap.  Immediately  the  spark  discharges,  part  of  the  electrostatic 
field  is  converted  into  current  and  the  remainder  into  a  wave  motion. 

The  wave  motion  consists  of  an  expanding  static  field    (as  shown  in  Fig.  99)   which  is 


88 


PRACTICAL  WIRELESS  TELEGRAPHY. 


accompanied  by  a  magnetic  field  both  being  radiated  at  right  angles  to  each  other  and  to  the 
direction  of  propagation.     (The  magnetic  field  for  a  single  cycle  of  current  in  the  oscillator  is 

shown  in  Fig.  100).  This  wave  motion  is  propagated 
through  space  at  a  velocity  of  186,000  miles  per  second 
corresponding  to  300,000,000  meters. 

At  a  distance  from  the  aerial  system  and  near  to  the 
earth  we  may  crudely  represent  the  fluxes  of  this  wave 
motion  by  the  crossed  arrows  of  Fig.  101,  where  the 
vertical  arrows  represent  the  electrostatic  field  and  the 
horizontal  arrows  the  magnetic  field.  If  these  two  fluxes 
act  upon  another  vertical  conductor  (or  elevated  ca- 
pacity) of  identical  natural  frequency  of  oscillation  as 
the  transmitter  aerial,  a  feeble  current  will  be  induced 

which  can  be  de- 
tected by  several 
methods.  If  means 
are  provided  t  o 

C  <^  ^  ^>^>^>          I      I      I       I  control  the  energy 

of  the  radiating 
station  in  the  form 
of  the  dots  and 
dashes  of  the  tele- 
graph code  and  if 
appropriate  d  e  - 
vices  are  supplied 
at  the  receiving 
station  whereby 
the  received  en- 
ergy can  be  inter- 
preted or  made 
audible,  we  then 
have  a  complete 
system  of  wireless 
telegraphy. 


Fig.     100 — Magnetic     Field    around     an 
aerial  for  three  alternations  of  current. 


Fig.    101 — Aerial   in   Path   of   Advancing 
Electromagnetic  Wave. 


91.  The  Length  of  the  Electric  Wave. — If  we  were  to  determine  the  length 
of  a  single  wave,  in  a  wave  motion  like  that  of  the  diagram  Fig.  102,  we  would 
simply  measure  the  physical  distance  between  two  points  in  the  successive  waves 
where  the  disturbance  is  at  a  maximum  or  a  minimum,  as  from  A  to  B  or  C  to  D, 
or  between  any  two  points  of  equal  disturbance.  Similarly  the  distance  between  two 

points  in  two  successive  electric  waves 
where  the  electrostatic  or  the  elec- 
tromagnetic field  is  maximum  and  in 
the  same  direction  would  be  the 
wave  length  of  the  electromagnetic- 
wave.  The  distance  from  A  to  B  in 
Fig.  99  represents  the  length  of  one 
complete  wave,  which  can  be  ex- 
pressed in  feet  or  in  meters.  The 
standard  zvave  lengths  of  commercial 


Fip. 


102. — Showing    .Veasurement   for   the    Length    of 
Single   Wave. 


radio-telegraphy  are  300,  450  and  600 
meters  corresponding  approximately  to  1,000,  1,500  and  2,000  feet. 

The  relation  between  the  velocity  of  electric  waves,  the  length  of  a  single  wave  and  the 
number  generated  per  second,  can  be  explained  as  follows:  Assume,  for  example,  that  1,000,- 
000  waves  pass  a  given  point  in  a  second  of  time  and  that  each  wave  is  of  300  meters  length  ; 
then  the  distance  through  which  the  first  wave  has  travelled  in  one  second  will  be  equal 
to  the  number  of  waves  multiplied  by  the  length  of  one  wave  or  100,000,000  X  300  or 
300,000,000  meters.  In  other  words,  the  distance  travelled  by  the  wave  motion  in  one  second 
is  the  velocity  of  the  wave.  This  may  be  written : 


THE 'RADIO  TRANSMITTER.  89 

Velocity  =  (Number  of  waves)  X  (Length  of  a  single  wave),  which  may  also  be  written: 

the  velocity  of  electric  waves 
Length  of  a  single  wave  =  — 

the  number  of  waves 

It  has  been  proven  by  experiment  that  the  velocity  of  electromagnetic  waves  is  practically 
the  same  as  the  velocity  of  light  which  may  be  taken  as  186,000  miles  per  second  or  the 
equivalent  of  300,000,000  meters.  Keeping  in  mind  that  a  single  cycle  of  current  in  an  open 
circuit  oscillator  sets  into  motion  a  single  electric  wave,  the  foregoing  formula  can  be  written : 

300,000,000 
Length  of  one  wave  =  — 

frequency  of  the  oscillations 
Hence  if  a  vertical  conductor  oscillates  at  a  frequency  of  500,000  cycles  per  second,  the  length 

300,000,000 
of  one  wave  in  the  resultant  wave  motion  will  be  —  —  =  600  meters.    - 

500,000 

If  the  wave  length  is  expressed  by  the  symbol  \  the  velocity  of  electric  waves  by  V  and 
the  frequency  of  the  oscillations  by  N,  the  formula  may  be  written : 

V 

X  =  — 
N 
A  table  of  standard  wave  lengths  corresponding  to  various  oscillation  frequencies  follows : 

Wave  Length  in  Meters.  Frequency  in  Cycles  per  Second. 

200  1,500,000 

300  1,000,000 

450  666,666 

600  500,000 

1000  300,000 

2000  150,000 

3000  100,000 

6000  50,000 

8000  37,500 

10000  30,000 

92.  The  Determination  of  Wave  Length  from  the  Inductance  and  Capacity. 

—The  length  of  the  wave  radiated  from  an  open  circuit  oscillator  can  be  calculated 
directly  from  knowledge  of  the  effective*  inductance  and  the  effective  capacity 
of  the  aerial.  The  formula  may  be  written : 

Wave  length  =  38  X  V~L~C~ 

Where  L  —  the  inductance  in  centimeters ; 

And  C  =  the  capacity  in  microfarads. 

If  an  inductance  coil  is  introduced  at  the  base  of  an  aerial,  the  formula  must 
be  modified  to  read : 

6.2832 

Wave  length  = X  9.5  X  V  L  C 

K 
Where  K  =  a  certain  correction  factor  (see  appendix). 

The  wave  length  of  a  straight  vertical  aerial  of  the  type  indicated  in  Fig.  97a  can  be 
determined  approximately  from  the  physical  dimensions.  The  length  of  the  wave  radiated 
from  a  grounded  vertical  oscillator  is  found  to  be  approximately  4.3  to  4.5  times  its  length. 
Therefore  if  the  vertical  wire  of  Fig.  97a  is  100  ft.  in  length,  a  single  electric  wave  would  have 
length  of  4.3  X  100  =  430  ft.  and  since  1  meter  =  3.25  feet,  the  wave  length  is  approximately 
132  meters.  If  the  aerial  consists  of  several  wires  connected  in  multiple,  the  factor  4.3  does 
not  apply  owing  to  the  increase  in  the  capacity  of  the  aerial  system.  However,  in  the  case  of 
a  4  wire  aerial,  the  wires  being  spaced  about  21/?  ft.  apart,  we  may  assume  the  factor  of  4.4  to 
4.8  which  multiplied  by  the  total  length  of  the  aerial  in  feet  or  meters  gives  an  approximation 
of  the  fundamental  wave  length. 

*The  effective  capacity  is  the  capacity  of  the  aerial  as  an  element  of  an  oscillation  circuit.  This 
value  varies  with  each  change  of  \. 


90 


PRACTICAL   WIRELESS   TELEGRAPHY. 


When  we  speak  of  the  wave  length  of  a  closed  oscillation  circuit  reference  is  made  to  the 
particular  frequency  at  which  the  circuit  oscillates  and  to  the  length  of  a  single  wave  in  the 
resultant  wave  motion  if  the  circuit  were  radiative.  Thus  if  the  frequency  of  a  given  closed 
oscillation  circuit  is  500,000  cycles  per  second,  it  would  correspond  in  the  case  of  an  open 
circuit  oscillator  to  a  wave  length  of  600  meters.  Hence,  we  would  state  that  the  wave  length 
of  the  closed  oscillation  circuit  is  600  meters. 

The  wave  length  of  a  closed  oscillation  circuit  may  be  computed  as  follows : 
Wave  length  =  59.6  X  V~L~C~ 

Where  L  =  The  inductance  of  the  circuit  in  centimeters ; 
And  C  =  The  capacity  of  the  condenser  in  microfarads. 

We  see  from  this  that  if  a  given  closed  oscillation  circuit  had  inductance  of 
10,000  centimeters  and  capacity  of  .01  microfarads,  the  wave  length  would  be 
59.6  X  V~l66~  =  596  meters. 


93.  Logarithmic  Decrement  of  the  Oscillations. — When  electrical  oscilla- 
tions are  created  in  an  antenna  or 
other  circuit  by  means  of  condensers 
discharges,  each  electric  spark  dis- 
charge creates  a  train  of  oscillations 
which  die  away.  The  oscillations  are 
assumed  to  decay  away  according  to 
the  law  that  the  ratio  of  any  oscillation 
to  the  one  preceding  is  constant.  This 
constant  ratio  is  called  the  damping  of 
the  oscillation  and  the  Naperian  loga- 
rithm of  the  ratio  of  one  oscillation 
to  the  preceding  one  is  .called  the 
logarithmic  decrement. 


Fig.   103— Highly  Damped  Oscillations. 


Thus  in  the  group  of  oscillations  of  Fig.  103,  the  maximum  amplitudes  of 
successive  alternations  are  represented  by  A1,  A2,  A3,  A4,  etc.,  and 

A^=JA8  -  A*. 

A3      A5      A7  '  e 
In  terms  of  logarithms  we  may  write 


where  e  =  base  of  the  Naperian  system  of  logarithms  ; 

$  =  a  constant  termed  the  logarithmic  decrement. 


By  transposition 


3   -1     or  A' 


If  we'  assume  that  the  oscillations  in  a  circuit  are  extinguished  when  the  ampli- 
tude of  the  last  oscillation  is  .01  of  the  initial  oscillation,  the  complete  number  of 
oscillations  (M)  in  a  spark  discharge  can  be  computed  as  follows: 


log 


M=- 


A* 


If  we  denote  the  last  oscillation  by  Ax  then  —  =  100,  and  since  the  Logarithm  of  100  =  4.605 
then 


THE   RADIO   TRANSMITTER. 


91 


Hence  in  a  group  of  oscillations 
Where  5  =  0.1 

4.605  +  0.1 


M  = 


0.1 


=  47  complete  oscillations. 


The  tuning  qualities  of  a  train  of  electric  waves  from  any  given  transmitting  station  depend 
greatly  upon  the  decrement  of  the  oscillations  and,  therefore,  the  determination  of  the  quantity 

is  an  important  measurement.    It  is  found 

-Feebly   Damped   Oscillations.  by  experiment  that  a  transmitter  having 

less  than  24  complete  oscillations  in  the 
antenna  circuit  per  single  spark  discharge, 
possesses  undesirable  tuning  qualities  and 
will  interfere  with  the  operation  of  other 
radio  stations  not  tuned  to  the  same  wave 
length. 

A  group  of  24  complete  oscillations 
corresponds  to  a  decrement  of  0.2  for  a 
complete  oscillation  which  is  the  arbitrary 
figure  enforced  by  U.  S.  Statutes.  This 
means  that  if  the  oscillations  in  a 
given  antenna  circuit  have  a  decrement 

A* 

of  0.2,  the  logarithm  of  the  ratio  of  — 

A3 

will  be  0.2.  (The  number  corresponding 
to  the  logarithm  0.2  is  1.2,  hence  the  ratio 

A1 

of  —  must  be  1.2  for  a  decrement  of  0.2.) 
AS 

A  group  of  oscillations  of  feeble 
damping  are  shown  in  Fig.  104;  a 
highly  damped  group  in  Fig.  105. 
If  the  conditions  of  damping  de- 
picted in  Fig.  104  apply  to  an  open 
circuit  oscillator  at  any  given  radio  station,  the  energy  of  the  radiated  wave  is 
largely  confined  to  a  single  frequency  and  the  effect  at  the  receiving  apparatus  is 
termed  in  operators'  language  "sharp  tuning"  meaning  that  very  careful  adjust- 
ment is  required  at  the  receiving  station  to  place  the  receiving  apparatus  in 
resonance  with  the  distant  transmitter,  i.  e.,  the  transmitting  apparatus  is  said 
to  be  "sharply  tuned."  But  with  the  conditions  of  Fig.  105  the  transmitter  is 
" broadly  tuned"  and  the  radiated  wave  said  to  have  "excessive  damping" 

These  trade  expressions  are  more  or  less  comparative  because  the  damping  of 
the  receiver  is  not  taken  into  account.  However,  the  terms  may  be  applied  in  a 
practical  way.  In  any  given  transmitter,  oscillations  of  great  amplitude  or 
strength  but  of  feeble  damping  are  really  desired,  but  in  certain  types  of  ap- 
paratus the  conditions  favorable  to  one  offset  the  conditions  favorable  to  the 
other;  hence  we  are  required  to  effect  a  compromise. 

The  exact  meaning  of  the  logarithmic  decrement  of  the  oscillations  in  radio-telegraphy 
may  be  better  understood  by  considering  the  oscillating  movements  of  a  pendulum  in  me- 
chanics. Suppose  for  example  a  plumb  bob  is  attached  to  the  end  of  a  string  and  suspended 
freely  as  in  Fig.  105a ;  drawn  to  one  side  and  released,  the  bob  will  vibrate  to  and  fro  until 
the  oscillation  is  completely  damped  out.  If  a  piece  of  cardboard,  for  instance,  were  attached 
to  the  string,  the  oscillations  of  the  pendulum  would  come  to  a  stop  in  a  much  shorter  period, 
e.  g.,  they  would  be  said  to  be  highly  damped.  This  corresponds  to  the  damping  of  the  oscil- 
lations in  a  radio  oscillation  circuit  having  considerable  resistance. 

Now  the  time  period  of  one  complete  oscillation  of  the  pendulum  can  be  obtained  by 
counting  the  oscillations  for  one  minute  and  dividing  their  number  by  60.  Hence  if  60  corn- 


Fig.   105 — Oscillations  with  Excessive  Damping. 


92 


PRACTICAL   WIRELESS    TELEGRAPHY. 


plete  oscillations  took  place  per  minute,  the  time  period  of  one  oscillation  would  be  one 
second.  This  could  he  easily  arranged  if  the  string  of  the  pendulum  had  a  certain  length  and 
the  bob  a  certain  weight. 

Assume  that  the  bob  oscillated  in  front  of  a  scale  calibrated  in  inches  as  in  Fig.  105a  and 

that  at  the  beginning  of  the  first  oscillation 
it  was  drawn  from  the  zero  position  out- 
ward to  a  distance  of  15  inches.  On  ac- 
count of  wind  friction,  at  the  end  of  one- 
half  oscillation  the  bob  would  not  have 
travelled  15  inches  from  the  zero  position. 
Under  suitable  conditions  it  would  move 
outward,  let  us  say,  14.2  inches  and  in  the 
return  swing,  its  movement  would  be  still 
less,  say  13.5  inches.  That  is,  the  swing  of 
the  second  oscillation  would  be  \l/2  inches 
less  than  the  initial  oscillation. 

If  we  observe  the  length  of  the  succes- 
sive swings  and  plot  the  results  as  in  Fig. 
104,  we  would  obtain  a  group  of  decaying 
oscillations  which  would  bear  the  following 
ratio  to  one  another.  If  the  horizontal  axis 
A,  B  is  divided  into  seconds,  and  the  ampli- 
tude of  the  right  and  left  swings  of  the 
pendulum  shown  by  -the  notations  on  the 
vertical  axis,  (0  to  15,  in  either  direction) 
then  the  amplitude  at  the  termination  of  a 
complete  period  or  at  any  partial  period  of 
oscillation  will  be  as  follows :  at  the  end 


Fig.   105a — Pendulum  for  Illustrating  Damping. 


of  the  first  half  period,  the  amplitude  of  the  oscillation  will  be  14.2;  at  the  end  of  a  complete 
period,  13.5.  After  the  pendulum  has  been  in  oscillation  for  \y2  seconds,  the  amplitude  will 
be  12.8  and  at  the  end  of  two  seconds,  12.1  and  so  on. 

It  is  to  be  especially  noted  that  the  ratio  of  the  amplitude  of  the  successive  oscillations  is 
constant,  e.  g.,  15/13.5  =  1.11;  13.5/12.1  =  1.11,  and  so  on.  This  ratio  will  be  constant  to  the 
end  of  the  swings  of  the  pendulum  because  it  is  a  law  of  all  mechanical  oscillating  motions, 
that  the  oscillations  will  die  away  at  such  rates  that  the  ratio  of  successive  amplitudes  remains 
constant.  This  is  the  condition  assumed  to  exist  in  the  spark  discharge  circuits  of  radio 
transmitters. 

It  should  now  be  clear  that  if  the  friction  to  the  oscillation  of  the  pendulum  were  increased 
by  attaching  a  cardboard  damper  to  the  pendulum,  the  swings  would  die  out  more  rapidly  and 
the  ratio  of  the  successive  amplitudes  would  take  a  higher  figure.  The  damping  of  the 
oscillations  would  thus  be  increased.  We  see  from  this  that  slow  damping  corresponds  to 
a  great  number  of  swings  of  the  pendulum  whereas  rapid  damping  corresponds  to  a  small 
number  of  swings  of  the  pendulum. 

Now  the  ratio  of  the  amplitude  two  successive  oscillations  is  called  the  damping  factor 
and  the  logarithm  of  this  ratio  is  called  the  logarithmic  decrement.  In  the  problem  cited  the 
ratio  is  1.11  and  if  reference  is  made  to  any  table  of  Naperian  logarithms,  the  logarithm  of 
1.11  will  be  found  to  be  approximately  0.10.  Similarly  if  the  ratio  were  1.22,  the  logarithmic 
decrement  would  be  0.2,  the  limit  allowed  by  law. 

The  decrement  of  the  oscillations  flowing  in  the  aerial  of  a  modern  radio-transmitter  is 

4.605  +  .05 

frequently  .05  per  complete  cycle,  hence  for  each  spark  there  will  be or  92  complete 

.05 
oscillations. 

It  is  now  apparent  that  the  feebler  the  decrement,  the  greater  will  be  the  number  of 
oscillations  per  spark  and  provided  their  amplitude  does  not  fall  too  much,  the  stronger  will 
be  the  current  induced  in  the  receiving  aerial  per  spark  of  the  transmitter.  This  combined 
with  the  favorable  tuning  qualities  of  a  feebly  damped  transmitter  brings  about  very  desirable 
conditions. 


THE   RADIO   TRANSMITTER. 


93 


The  damping  of  a  circuit  of  a  radio-frequency  depends  upon  the  effective  capacity,  in- 
ductance and  resistance  of  that  circuit.    These  quantities  are  related  in  the  following  manner : 

5  =  1.57  X  R1  X  t/£- 

Where  R1  =  High  frequency  resistance  in  ohms, 
C  —  Capacity  in  farads, 
L  =  Inductance  in  henries, 
8  =  Decrement  per  semi-oscillation. 
C 
The  ratio  of  —  changes  the  values  of  R1  but  we  may  state  generally  that  increase  of  L  or 

L 

decrease  of  C  will  reduce  the  damping  of  the  oscillations,   e.  g.,   decrease  the  logarithmic 
decrement. 

Assume  a  circuit  of  .ol  microfarads  capacity,  inductance  of  10,000  centimeters  and  resistance 
of  3  ohms,  then 


5  =  1.57  X  3 


f   .00000001 

x  V 


=  0.148 


.00001 


Practical  measurement  of  the  logarithmic  decrement  will  he  described  in  Part  XI. 

94.  Methods  of  Exciting  Oscillations  in  an  Aerial. — The  antenna  circuit 
can  be  set  into  oscillation  in  ordinary  spark  telegraphy  in  several  different  ways 
as  follows : 

(1)  By  direct  excitation  (plain  aerial  connection); 

(2)  By  inductive  coupling  to  a  closed  oscillation  circuit; 

(3)  By  conductive  coupling  to  a  closed  oscillation  circuit; 

(4)  By  capacitive  coupling  to  a  closed  oscillation  circuit; 

Direct  excitation  of  the  aerial  has  been  shown  in  Fig.  97a,  where  the  spark 
gap  is  connected  directly  in  series  with  the  antenna  circuit.  The  winding  S  is 
the  secondary  of  an  induction  coil  wherein  the  pressure  of  the  current  is  several 
thousand  volts.  This  coil  is  generally  fitted  with  a  magnetic  interrupter  and 
energized  by  a  15  to  30  volt  storage  battery. 

The  advantage  of  this  method  of  excit- 
ing oscillations  in  the  aerial  lies  in  the  sim- 
plicity of  the  connections  but  there  are  sev- 
eral objections  to  its  use.  The  principal 
objection  is  the  fact  that  the  oscillations 
are  rapidly  damped  out  and  the  waves 
radiated  from  the  aerial  will  interfere  with 
the  reception  of  signals  at  other  stations 
even  when  not  tuned  to  the  frequency  of 
this  transmitter.  The  second  disadvantage 
is  that  the  insulation  of  the  aerial  is  sub- 
jected to  an  abnormal  strain  caused  by  the 
high  voltage  necessary  and  by  the  low  fre- 
quency current  of  the  induction  coil  being 
superposed  on  the  radio-frequent  oscilla- 
tions. 

The  oscillations  in  the  antenna  circuit  of 
the  plain  aerial  transmitter  will  be  damped 
out  less  rapidly  if  fair  amounts  of  induct- 
ance are  inserted  at  the  base  as  at  L,  Fig. 
97b.  Under  these  conditions,  the  decrement 
of  the  oscillations  may  compare  favorably 
with  modern  methods  of  excitation. 

Indirect  methods  of  antenna  excitation  are  shown  in  Figures  106,  107  and  108.  The 
principal  advantage  of  these  methods  of  coupling  lies  in  the  fact  that  the  closed  circuit  con- 
denser acts  as  a  reservoir  of  energy  and  since  the  capacity  of  the  condenser  in  the  closed 
oscillation  circuit  is  generally  several  times  that  of  the  capacity  of  the  aerial,  it  permits  the 
closed  circuit  to  use  large  amounts  of  power  for  the  same  wave  length,  spark  frequency  and 


•©©• 


\  106-   Inductively  Coupled  Transmitter. 


94 


PRACTICAL   WIRELESS   TELEGRAPHY. 


voltage  which  results  in  the  production  of  more  powerful  oscillation.  Then  by  proper  adjust- 
ment of  coupling,  these  oscillations  may  be  transferred  to  the  antenna  systems  at  a  certain 
rate  and  radiated  in  the  form  of  electro-magnetic  waves. 

Inductive  coupling  of  the  open  and  closed  oscillation  circuits  is  shown  in  the 
diagram.  Fig.  106.  The  spark  gap  S  is  now  placed  in  the  closed  oscillation 
circuit,  the  oscillations  generated  in  the  latter  are  transferred  to  the  antenna 
through  the  oscillation  transformer,  L,  L-l. 

When  the  condenser  discharges  through  L,  the  lines  of  force  cut  through  L1, 
setting  up  in  the  aerial  circuit  oscillations  of  similar  frequency  provided  the  aerial 
circuit  is  carefully  tuned  to  the  closed  circuit.  Winding  L  is  seen  to  serve  the 
double  purpose  of  governing  the  frequency  of  the  oscillations  in  the  condenser 
circuit  and  transferring  them  by  magnetic  induction  to  the  aerial  circuit. 

If  L  and  L1  are  closely  coupled,  the  oscillations  in  the  aerial  system  will  be  damped  out 
rapidly  due  to  a  part  of  the  energy  being  retransferred  back  to  the  spark  circuit,  but  if  L 
and  L1  are  loosely  coupled  (drawn  apart)  the  oscillations  in  the  aerial  circuit  will  be  damped 

out  less  rapidly.    We  see  from  this  that  the 
ALRIAL      \    I    /     tuning1    qualities    of    the    transmitter    are 
\    /      largely   controlled   by   the   coupling   of   the 
^        oscillation  transformer.     It  is  usual  to  ad- 
just the  coupling  to  a  degree  that  will  give 
oscillations   of.  fairly   feeble   damping  pro- 
vided the  flow  of  the  current  in  the  an- 
tenna    circuit     is     not     seriously     reduced 
thereby. 


Conductive  coupling  of  the  closed 
and  open  oscillation  circuits  is  shown 
in  Fig.  107,  where  an  auto  transform- 
er is  employed  to  transfer  the  oscilla- 
tions to  the  antenna.  In  this  diagram 
the  turns  from  A  to  B  constitute  the 
primary  winding  and  the  turns  from 
C  to  D,  the  secondary  winding.  When 
condensers  C-l,  C-2,  discharge 
through  winding  P,  an  alternating 
magnetic  field  threads  through  S, 
which  follows  the  primary  current. 
Oscillations  of  radio-frequency  are  thus  induced  in  the  secondary  circuits.  The 
advantage  of  this  method  of  coupling  lies  in  the  simplicity  afforded  by  the  use 
of  a  single  helix  for  transforming  the  oscillations.  The  inductively  coupled  sys- 
tem, on  the  other  hand,  requires  a  primary  and  secondary  helix  with  the  necessary 
mechanical  arrangements  for  adjust- 
ment of  the  coupling.  There  is  little 
difference  in  the  degree  of  efficiency 
obtained  by  these  two  methods  of 
coupling;  but  the  coupling  between 
the  closed  and  open  oscillation 
circuits  can  be  more  easily  adjusted 
by  the  inductive  transformer  than 
by  the  conductive  transformer.  If 
the  earth  connection  to  the  helix 
in  Fig.  107  is  fitted  with  a  contact 
clip,  the  turns  included  from  C  to  D 
can  be  placed  at  a  distance  from  the 
turns  in  use  between  A  and  B  and  in 
this  manner  the  coupling  between  the  F:g.  ios— One  Method  of  Capacitive  Coupling. 


Fig.  107 — Conductively  Coupled  Transmitter. 


THE   RADIO   TRANSMITTER. 


95 


two  circuits  can  be  closely  adjusted,  but,  of  course,  with  greater  difficulty  than 
in  the  inductive  system. 

In  the  capacitive  coupling  shown  in  Fig.  108,  a  condenser  C-2  is  connected 
in  series  with  the  aerial  system  and  is  also  placed  between  the  plates  of  the 
condenser  C-l,  which  is  a  part  of  the  closed  oscillation  circuit.  Part  of  the  elec- 
trostatic field  of  C-l  is  transferred  to  C-2  and  accordingly  oscillations  will  flow 
in  the  aerial  circuit.* 

The  majority  of  radio  stations  employ  the  inductive  method  of  coupling. 

95.  The  Reaction  of  Coupled  Circuits. — We  might  expect  that  when  the 
open  and  closed  oscillation  circuits  of  a  radio-telegraph  transmitter  are  adjusted 
independently  to  a  frequency  corresponding  to  a  wave  length  of  600  meters  and 
afterwards  the  circuits  are  coupled  for  the  transference  of  energy,  the  radiated 
wave  would  be  of  the  same  length  but  this  may  not  be  the  case.  Under  certain 
conditions  of  coupling,  oscillations  of  two  frequencies  occur  in  the  antenna  circuit. 
For  example,  if  the  closed  and  open  circuits  are  set  for  the  wave  of  600  meters,  we  may  find 
(by  means  of  a  wavemeter)  two  waves  being  radiated  from  the  aerial,  one  630  meters  in 
length  and  the  other  570  meters  in  length,  corresponding  to  two  distinct  frequencies  of  oscilla- 
tion. These  two  frequencies  are  caused  by  the  reaction  of  the  magnetic  field  of  the  open 
circuit  upon  the  closed  circuit,  and  vice  versa,  that  is,  the  oscillations  flowing  to  and  fro 
through  the  primary  circuits  induce  currents  in  the  secondary  circuit  and  the  magnetic  field 
of  the  secondary  currents  tends  to  react 

and    induce    currents    in    the    primary.  PRIM- 

This  interchange  of  energy  goes  on  un- 
til the  current  in  the  primary  can  no 
longer  bridge  the  spark  gap  when  the 
process  stops.  This  interlinkage  of  the 
lines  of  force  of  the  two  windings  causes 
the  effective  self-induction  of  the  an- 
tenna coil  either  to  increase  or  decrease 
according  to  which  of  the  two  circuits 
is  driving  the  other,  and  results  in  the 
production  of  two  sets  of  oscillations. 
The  resulting  oscillations  of  coupled 
circuits  may  be  shown  by  the  dia- 
gram of  Fig.  109,  where  two  groups  of 
oscillations  in  the  closed  and  open  cir- 
cuits are  represented.  It  will  be  noted 
that  the  oscillations  in  the  closed  cir- 
cuit have  maximum  amplitude  when 
those  in  the  open  circuit  have  minimum 
amplitude,  and  vice  versa.  In  the  sense  of  a  second  of  time  the  two  sets  of  oscillations  take 
place  simultaneously,  but  in  the  sense  of  a  fraction  of  a  second  they  do  not  reach  their 
maximum  amplitude  simultaneously. 

When  the  antenna  oscillates  at  two  distinct  frequencies,  two  waves  of  different  length  are 
set  into  motion.  This  is  an  undesirable  condition,  (1)  because  the  receiving  apparatus  gen- 
erally can  be  tuned  only  to  one  of  the  radiated  waves,  the  energy  of  the  other  being  lost; 
(2)  a  needless  amount  of  interference  is  caused  thereby  to  the  operation  of  other  radio 
stations. 

The  true  coefficient  of  coupling  between  two  circuits  of  this  character  is  determined 
by  the  following  formula: 

M 


Fig.  109 — Showing  Oscillations  in  Primary  and  Secondary 
of  Coupled  Transmitter. 


where  L1  and  L9  —  self-inductance  of  the  primary  and  secondary  circuits  respectively, 

M  =  coefficient  of  coupling. 

It  is  the  custom   to  determine  the  coupling  from  actual  measurement   of  the  radiated 
waves,  or 


*This    is    coupling   by    electrostatic    induction.      Direct    or    conductive    electrostatic    induction    may    also 
be  used. 


96 


PRACTICAL   WIRELESS   TELEGRAPHY. 


X22  —  X2i 

X'j  -f  X2i 
\Vhere  \*  —  longer  wave, 

Xi  — shorter  wave. 

Also  if  Xs  equals  the  wave  length  to  which  the  primary  and  secondary  circuits  are  ad- 
justed independently,  then 

X2  =  Xs  V~~l  +  K 
Xi  -  X3  V  1  —  K 

In  other  words,  if  the  value  of  the  coupling  K  is  known,  the  two  wave  lengths  can 
be  calculated  before  actual  measurement. 

When  the  primary  winding  of  the  oscillation  transformer  is  placed  close  to  the  second- 
ary winding,  perhaps  partially  telescoped  into  it,  the  set  is  said  to  have  "tight  coupling"  or 
"close  coupling,"  but  when  these  two  windings  are  drawn  apart  the  coupling  is  said  to  be 
"loose." 

Now  if  oscillations  of  two  frequencies  flow  in  an  aerial  system,  and  afterward  the 
coupling  is  gradually  reduced,  the  two  sets  of  oscillations  gradually  merge  into  oscillations 
of  a  single  frequency  and  generally  with  this  adjustment,  the  radiated  wave  is  feebly 
damped  or  of  low  decrement ;  i.  c.,  it  has  the  damping  of  the  antenna  circuit. 

The  student  should  note  this  carefully:  With  the  plain  spark  discharger  in  the  closed 
circuit,  "close  coupling"  of  the  primary  and  secondary  windings  generally  causes  a  "broad 
•wave"  to  be  radiated  from  the  aerial  system,  while  "loose  coupling"  results  in  the  radiation 
of  a  "sharp  wave."  The  terms  are,  of  course,  merely  relative.  (See  Appendix,  Section  G.) 


o    . 
Q  I 


T-2 


FOR  300  METER5  OPEN 
5-1    AND   REDUCE  INDUCTANCE 
AT    T-i     T-2       \ 


Fig.   110 — Diagram  Showing  Changes  Necessary  to  shift  from  600  meter  to  300  meter  Wave. 

96.  The  Standard  Waves  of  Commercial  Wireless  Telegraphy.— The  rules 
of  the  International  Radio-Telegraphic  Convention  require  the  use  of  tivo  stand- 
ard waves  for  the  dispatch  of  commercial  wireless  traffic,  namely  300  and  600 
meters.  Either  of  these  waves  must  be  employed  for  calling  another  station  but 


THE    RADIO   TRANSMITTER.  97 

after  communication  is  established,  waves  of  any  length  between  300  and  600 
meters  may  be  used.  The  rules  also  specify  that  by  special  license  vessels  may 
employ  waves  in  excess  of  1600  meters. 

For  ordinary  marine  traffic,  naval  stations  of  the  United  States  employ  waves 
varying  in  length  from  600  to  1600  meters.  High  power  naval  and  commercial 
stations  use  waves  varying  from  1600  meters  to  10,000  meters.  The  United 
States  regulations  limit  amateur  stations  to  the  use  of  a  200  meter  wave  with 
power  input  of  1  K.  W.  If  the  amateur  station  is  located  within  five  miles  of  a 
naval  station  their  sets  are  limited  to  an  input  of  ^2  K.  W. 

The  standard  marine  radio  sets  of  the  Marconi  Wireless  Telegraph  Company 
of  America  vary  in  power  from  y$  K.  W.  to  2  K.  W.  and  are  designed  for  three 
waves  of  300,  450  and  600  meters  length. 

The  average  ship's  aerial  in  the  Marconi  service  has  a  fundamental  wave  length  of  325 
meters,  capacity  of  approximately  of  .'001  microfarads  and  inductance  of  70,000  centimeters.  In 
order  that  the  standard  waves  of  300  and  600  meters  may  be  radiated,  an  extra  inductance 
must  be  included  at  the  base  for  the  600  meter  wave  and  a  condenser  must  be  connected  in 
series  for  the  300  meter  wave.  This  coil  and  condenser  are  called:  (1)  the  aerial  tuning 
inductance;  (2)  the  short  wave  condenser.  The  aerial  tuning  inductance  various  from 
20,000  to  80,000  centimeters  inductance  and  the  short  wave  condenser  usually  is  of  fixed 
capacity,  approximately  .0005  microfarads.  The  position  occupied  by  these  devices  in  the 
antenna  circuit  is  shown  in  Fig.  110.  The  condenser  is  shunted  by  a  switch  S-l  which  is 
closed  for  the  600  meter  wave.  The  tuning  inductance  L-3  is  fitted  with  a  plug  contact 
permitting  one  turn  or  a  fraction  of  a  turn  to  be  inserted  in  the  circuit  at  A1. 

The  method  of  shifting  from  one  standard  wave  to  the  other  in  a  commercial 
set  is  shown  in  Fig.  1 10.  Assume  the  open  and  closed  circuits  to  be  set  for  the 
600  meter  wave :  Then  if  the  circuits  are  to  be  adjusted  for  300  meters,  the 
following  procedure  is  involved: 

(1)  The  variable  contact  T-l  at  the  primary  winding  of  the  oscillation  trans- 
former is  set  at  a  lesser  value  of  inductance  (as  indicated  by  the  dotted 
lines) ; 

(2)  Similarly  the  variable  contact  of  the  aerial  tuning  inductance  L-3; 

(3)  The  switch  S-l  shunting  the  short  wave  condenser  is  opened. 

If  the  condenser  of  the  closed  circuit  exceeds  .015  microfarads  capacity,  its  capacity  must 
be  reduced  for  the  300  meter  wave,  to  say,  .006  microfarads. 

When  a  condenser  is  connected  in  series  with  an  open  circuit  oscillator,  the  total 
capacity  is  affected  just  as  it  is  when  two  ordinary  condensers  are  connected  in  series, 
i.  e.,  the  capacity  is  reduced  and  the  resulting  value  will  be  less  than  the  capacity  of  the 
smallest  condenser  in  the  circuit.  We  have  already  shown  that  the  wave  length  of  an 
open  circuit  oscillator  =  38  V  LC ;  hence  a  decrease  of  the  value  of  C  will  decrease  the 
length  of  the  radiated  wave. 

We  may  then  write  for  the  open  circuit  oscillator, 

-L    CC- 


c  +  c3 

where  C3  —  capacity  of  the  short  wave  condenser. 

When  an  extra  coil  of  inductance  is  inserted  at  the  base  of  an  aerial  the  formula  becomes 

6.28 

X  = X  9.5  V  LC 

K 
Where  K  =  a  certain  correction  factor,  the  ratio  of  the  inductance  of  the  coil  to  the 

inductance  of  the  aerial  ; 

L  =  total  effective  inductance  of  the  aerial  in  centimeters ; 
C  =  capacity  of  the  aerial  in  microfarads. 

If  very  large  values  of  concentrated  inductance  are  inserted  at  the  base  of  the  aerial, 
the  correction  factor  may  for  all  practical  purposes  be  ignored. 

97.  Fundamental  Circuit  of  a  Complete  Radio  Transmitter. — We  have 
shown  how  alternating  current  can  be  obtained  from  a  generator  and  how  it  can 
be  raised  to  a  pressure  of  several  thousand  volts  by  a  step-up  transformer.  It 
has  been  explained  that  this  current  of  high  pressure  can  be  stored  up  temporarily 


98 


PRACTICAL  WIRELESS  TELEGRAPHY 


THE   RADIO   TRANSMITTER.  99 

in  a  condenser  and  discharged  through  the  closed  circuit  in  the  form  of  radio- 
frequent  oscillations,  and  that  these  oscillations  can  be  transferred  to  an  aerial 
circuit,  where  a  part  of  their  energy  is  radiated  in  the  form  of  an  electromagnetic 
wave.  We  see,  then,  that  the  principal  parts  of  a  radio-transmitter  are : 

(1)  The  alternating  current  generator; 

(2)  The  alternating  current  step-up  transformer; 

(3)  The  condenser; 

(4)  The  oscillation  transformer; 

(5)  The  aerial  or  antenna; 

(6)  The  short  wave  condenser; 

(7)  The  aerial  tuning  inductance; 

(8)  The  transmitting  key. 

(9)  The  spark  gap. 

The  student  should  now  focus  his  attention  on  the  diagram  of  Fig.  Ill  where 
direct  current  enters  the  motor  at  the  left  of  the  drawing  but  eventually  at  the 
extreme  right,  an  alternating  current  of  radio-frequency  flows  in  the  antenna 
circuit. 

A  complete  transmitting  set  consists  of  the  following  apparatus: 

(1)  A  motor  generator  to  convert  direct  current  to  alternating  current. 

(2)  A  starting  box  to  regulate  the  flow  of  current  through  the  motor  armature 
during  the  starting  period. 

(3)  Field  rheostats  to  regulate  the  frequency  and  the  voltage  of  the  generator. 

(4)  A  step-up  transformer  to  raise  the  voltage  of  the  alternating  current  to  a 
value  in  excess  of  10,000  volts. 

(5)  An  ammeter  to  measure  the  current  flowing  through  the  transformer. 

(6)  A  voltmeter  to  measure  the  voltage  at  the  terminals  of  the  alternator. 

(7)  A  wattmeter  to  measure  the  power  input  to  the  transformer. 

(8)  A  reactance  regulator  to  regulate  the  flow  of  current  through  the  primary 
winding  of  the  transformer. 

(9)  A  telegraph  key  to  permit  the  current  to  be  interrupted  in  the  form  of  the 
dots  and  dashes  of  the  Morse  code. 

(10)  A  battery  of  condensers  for  the  production  of  radio-frequent  oscillations. 

(11)  A  spark  discharge  gap  to  discharge  the  energy  stored  up  in  the  condenser. 

(12)  An  oscillation  transformer  to  transfer  current  at  radio  frequencies  from  the 
condenser  circuit  to  the  aerial  circuit. 

(13)  An  aerial  ammeter  to  determine  conditions  of  resonance  between  the  con- 
denser circuit  and  the  aerial  circuit. 

(14)  A  short  wave  condenser  to  decrease  the  time  period  of  oscillation  of  the 
antenna  circuit. 

(15)  An  aerial  tuning  inductance  to  increase  the  time  period  of  oscillation  of 
the  antenna  circuit. 

(16)  An  aerial  change-over  switch  to  shift  the  aerial  alternately  from  the  trans- 
mitting apparatus  to  the  receiving  apparatus. 

(17)  An  aerial  to  radiate  energy  in  the  form  of  electromagnetic  waves. 

98.  Simple  Explanation  of  the  Circuits. — A  simple  explanation  of  the 
functioning  of  this  apparatus  follows:  Direct  current,  at  pressure  of  110  volts, 
enters  the  motor  armature  through  the  starting  box  and  sets  it  into  rotation.  The 
alternator  in  turn  generates  alternating  current  at  pressures  varying  from  110  to 
500  volts  according  to  the  design  of  the  machine. 

When  the  telegraph  key  is  closed,  current  flows  from  the  generator  armature 
through  the  primary  winding  of  the  transformer  setting  up  magnetic  lines  of 
force  which  intersect  or  cut  through  the  secondary  winding  inducing  therein  a 
current  at  pressures  varying  from  10,000  volts  to  25,000  volts. 

Now  the  voltage  and  the  frequency  of  the  alternating  current  can  be  adjusted 
(by  the  generator  Held  rheostat  and  the  motor  field  rheostat.  For  example,  if  re- 
sistance be  added  at  the  motor  field  rheostat,  the  motor  will  increase  its  speed  and, 
accordingly,  the  frequency  of  the  generator  will  be  increased.  This  will  also  tend 
to  increase  the  voltage  of  the  generator.  If  resistance  be  added  at  the  generator 


100  PRACTICAL   WIRELESS   TELEGRAPHY. 

field  rheostat,  the  voltage  of  the  generator  armature  will  be  reduced  and  con- 
versely if  the  resistance  of  this  rheostat  is  decreased,  the  voltage  of  the  generator 
will  increase. 

The  terminals  of  the  secondary  winding  of  the  high  voltage  transformer  are 
connected  to  the  terminals  of  a  battery  of  condensers  where  the  energy  is  stored 
up  temporarily  in  the  form  of  electrostatic  lines  of  force.  When  the  limit  of 
charge  for  each  alternation  of  charging  current  has  been  reached,  the  condenser 
will  discharge  across  the  spark  gap  through  the  primary  winding  of  the  oscillation 
transformer,  the  discharge  consisting  of  a  number  of  radio-frequent  oscillations. 
The  frequency  of  the  oscillations  will  decrease  if  inductance  is  added  at  the 
primary  winding  or  increase  if  the  inductance  be  reduced.  An  increase  or  de- 
crease of  the  capacity  of  the  condenser  affects  the  frequency  in  the  same  manner, 
e.  g.,  a  reduction  of  capacity  will  increase  the  frequency  of  the  oscillations  while 
an  increase  of  capacity  will  reduce  the  frequency  of  the  oscillations. 

The  oscillations  flowing  in  the  closed  circuit  are  transferred  to  the  aerial  cir- 
cuit through  the  oscillation  transformer  and  a  portion  of  the  energy  is  radiated 
from  the  antenna  in  the  form  of  electromagnetic  waves.  In  a  simple  single  wire 
aerial  system,  the  wave  length  of  this  wave  motion  will  be  approximately  4.3 
times  the  linear  length  of  the  oscillator.  The  length  of  the  radiated  wave  in 
fact  varies  inversely  as  the  frequency  of  the  oscillation.  The  higher  frequencies 
such  as  500,000  and  1,000,000  cycles  per  second  correspond  to  the  shorter  waves 
such  as  600  meters  and  300  meters  respectively,  while  lower  frequencies  of  oscil- 
lation from  30,000  to  100,000,  for  example,  correspond  to  the  longer  waves  from 
10,000  meters  down  to  3,000  meters. 

If  a  receiving  aerial  be  erected  at  a  distant  station  and  its  natural  time  period 
of  oscillation  adjusted  to  the  frequency  of  the  oscillations  flowing  in  the  trans- 
mitter aerial,  feeble  currents  will  be  induced  in  it  which  by  appropriate  devices 
may  be  heard  in  the  receiving  telephone.  More  clearly,  the  fluxes  of  the  advancing 
wave  will  induce  currents  in  the  receiving  aerial  having  substantially  the  fre- 
quency of  the  oscillations  in  the  transmitting  aerial.  By  appropriate  devices 
within  the  station,  these  currents  can  be  translated  into  the  language  of  the  sender. 

99.  Numerical  Values  for  a  Standard  Radio  Set. — In  order  to  familiarize 
students  with  the  power  consumption,  the  capacity  and  inductance  of  the  various  elements 
in  a  standard  transmitting  set,  the  following  data  is  presented.  The  motor  of  the 
2  K.  W.  500  cycle  panel  transmitting  set  takes  4.3  H.  P.,  the  generator  delivers  2  K.  W. 
Current  enters  the  motor  armature  at  110  volts  D.  C.,  but  the  voltage  developed  at  the 
generator  armature  is  approximately  380  volts  on  open  circuit  and  approximately  120 
volts  when  the  transmitting  key  is  depressed.  The  frequency  of  the  generator  is  500 
cycles  (this  frequency  is  now  standard). 

The  transformer  is  of  the  closed  core  type;  the  secondary  potential  is  14.500  volts. 
The  capacity  of  the  condenser  for  the  450  and  600  meter  waves  is  .012  microfarads,  but  for 
the  300  meter  wave  it  is  reduced  to  .006  microfarads. 

The  primary  winding  of  the  oscillation  transformer  has  maximum  inductance  of  ap- 
proximately 10,000  centimeters  correspondingly  lesser  values  being  used  for  the  shorter 
waves.  The  secondary  winding  has  approximately  30,000  centimeters  while  the  two  aerial 
tuning  inductance  coils  have  combined  inductance  of  about  80,000  centimeters.  The  aerial 
ammeter  has  range  0-25  amperes,  the  wattmeter,  0-3  kilowatts. 

For  the  standard  }/2  K.  W.  set  the  motor  is  rated  at  1  H.  P.,  the  generator  at  ^  K.  W. 
The  voltage  at  the  transformer  is  about  15,000  volts  and  the  capacity  of  the  closed  circuit 
condenser  .004  microfarads.  The  inductance  of  the  primary  at  the  wave  of  600  meters  is 
approximately  25,000  centimeters.  Lesser  values  are,  of  course,  employed  for  the  300  and 
450  meter  waves.  The  secondary  inductance  would,  of  course,  be  the  same  as  with  the 
2  K.  W.  set. 


PART  VII. 
APPLIANCES  FOR  A  RADIO  TRANSMITTER 

SPARK  DISCHARGERS  —  OSCILLATION   TRANSFORMERS  —  CON- 
DENSERS— TRANSFORMERS. 

100.  IN  GENERAL.  101.  SPARK  DISCHARGERS  FOR  RADIO  TELEG- 
RAPHY. 102.  ADJUSTMENT  OF  THE  SPARK  NOTE.  103.  OSCIL- 
LATION TRANSFORMERS.  104.  AERIAL  TUNING  INDUCTANCE. 
105.  THE  SHORT  WAVE  CONDENSER.  106.  HIGH  POTENTIAL 
CONDENSERS.  107.  HIGH  FREQUENCY  "CHOKING"  COILS.  108. 
HIGH  VOLTAGE  TRANSFORMERS.  109.  REACTANCE  REGULATORS. 
110.  AERIAL  CHANGEOVER  SWITCH.  111.  TRANSMITTING  KEYS. 

100.  In   General. — Although   the   apparatus   of   the   radio  transmitter  has 
been  described  and  discussed  in  a  fundamental  way,  certain  important  parts  of 
the  complete  set  will  require  more  detailed  treatment.     Principal  among  these 
are  the  various  types  of  spark  gaps,  condensers,  oscillation  transformers,  sig- 
nalling keys,  etc.,  each  of  which  may  take  one  of  several  designs  or  may  differ 
in  their  mode  of  functioning. 

101.  Spark  Dischargers  for  Radio-Telegraphy. — The  functions  of  the  spark 
gap  in  a  radio-transmitter  arc:   (1)   to  keep  the  closed  oscillation  circuit  idle  until  the  con- 
denser is  fully  charged;  (2)  to  discharge  the  energy  stored  up  in  the  condenser  in  the  form 

of  radio-frequent  oscillations;  (3)  to 
quench  the  spark,  i.  e.,  to  restore  the  gap 
to  its  non-conducting  state,  when  the 
energy  has  once  been  transferred  to  the 
antenna. 

The  ideal  gap  would  be  one  of  in- 
finite resistance  during  the  charging 
period  of  the  condenser  but  of  zero 
Fig.  ii2-Simpie  Spark  Discharger.  resistance  during  discharge-a  con- 

dition partially  realized  in  the  rotary  gap  but  not  completely  fulfilled. 

The  transmitting  sets  of  the  Marconi  Company  employ  four  general  types  of 
spark  dischargers  as  listed  below : 

(1)  Plain  spark  discharger; 

(2)  Non-synchronous  rotary  discharger; 

(3)  Synchronous  rotary  discharger; 

(4)  Quenched  spark  or  multiple  plate  discharger. 

(a)  In  its  most  common  form  the  plain  spark  discharger  is  composed  of  two 
brass,  zinc,  or  copper  rods  about  l/4  or  ^  inch  in  diameter  slightly  rounded  at  the 
discharge  tips  and  mounted  on  an  insulated  base  as  in  Fig.  112,  appropriate 
means  being  supplied  whereby  the  length  of  the  gap  may  be  carefully  and  closely 
regulated.  By  careful  adjustment  of  the  condenser  capacity,  the  voltage  of  the 
generator,  and  the  length  of  the  gap,  a  single  spark  discharge  for  each  alternation 
of  the  charging  current  can  frequently  be  obtained,  but  it  is  somewhat  difficult  to 

NOTE.  The  student  should  understand  thoroughly  that  a  spark  gap  is  not  required  in  order  that 
the  condenser  discharge  may  generate  high  frequency  oscillations,  but  it  may  be  regarded  as  a  necessary 
evil.  It  is  self  evident  that  if  the  spark  gap  is  closed  during  the  charging  period  of  the  condenser  the 
transformer  is  placed  on  short  circuit. 


102 


,   PRACTICAL   WIRELESS   TELEGRAPHY. 


maintain  this  state' of  adjustment;  ill  fact,  in  the  majority  of  cases,  multiple  dis- 
charges are  fow^d/  ,"*  f.4: 

The  chief  difficulty  With  the  plain '•  gap  is  the  tendency  to  form  an  arc  with  consequent 
impurity  of  the  oscillation.  Various  methods  have  been  devised  to  prevent  the  gap  arcing 
such  as  the  use  of  several  gaps  in  series,  or  the  applying  of  an  air  blast  either  at  right 
angles  to  the  discharger  or  through  hollow  electrodes  to  the  center  of  the  spark  discharge. 
The  latter  method  is  particularly  successful  for  not  only  is  arcing  prevented  but  a  very- 
clear  spark  note  is  obtained  as  well.  Gaps  of  this  construction  are  generally  used  in 
transmitting  sets  employing  60  cycle  current  transformers  or  induction  coils  as  a  source 
of  high  voltage  current.  (See  Section  G,  Appendix.) 

(b)  The  non-synchronous  rotary  spark  discharger  generally  consists  of  a 
circular  disc  of  metal  or  some  insulating  material  fitted  with  8  or  10  metal  spark- 
ing points  equally  spaced  about  the  circumference,  the  disc  being  mounted  on  the 
shaft  of  a  direct  current  motor  with  a  speed  controlling  rheostat  connected  in 
series.  The  disc  revolves  between  two  stationary  spark  electrodes  which  can  be 

carefully  regulated  in  respect  to  the 
revolving  electrodes.  When  the  two 
stationary  electrodes  are  connected  in 
series  with  the  closed  oscillation  cir- 
cuit of  a  transmitter,  the  spark  will 
discharge  whenever  an  electrode  of 
the  disc  comes  opposite  the  station- 
ary electrodes  and  the  pitch  of  the 
note  will  vary  as  the  speed  of  the 
motor. 

The  non-synchronous  rotary  dis- 
charger is  usually  fitted  to  60 
cycle  alternating  current  transmitting 
sets,  to  produce  a  spark  discharge  of 
musical  pitch,  but  it  may  be  employed 
with  alternating  current  transmitters 
of  any  frequency  as  well. 
The  construction  of  the  non-synchronous  gap  is  shown  in  the  drawing,  Fig.  113,  where 
an  insulated  disc  D  is  mounted  on  the  shaft  of  the  motor  M.  Equally  spaced  about  the 
periphery  are  the  spark  electrodes,  E,  E,  which  extend  through  the  disc.  Two  stationary 
electrodes,  D1,  D1,  are  mounted  on  a  base 
and  are  connected  to  the  condenser  and 
inductance  as  shown.  At  the  moment  one 
of  the  electrodes  on  the  disc  comes  be- 
tween the  stationary  electrodes,  the  spark 
will  discharge  and  owing  to  the  equal 
spacing  of  the  spark  points,  the  spark  note 
partakes  of  a  musical  pitch.  Experiments 
with  this  gap  indicate  that  the  most  ef- 
fective results  are  obtained  (with  60  cycle 
current)  when  the  disc  is  driven  at  a 
speed  that  will  give  from  200  to  300  spark 
discharges  per  second.  The  discs  are  fitted 
with  the  correct  number  of  spark  elec- 
trodes to  give  a  spark  of  this  frequency 
when  driven  by  a  motor  at  speeds  varying 
from  1,800  to  2,400  revolutions  per  minute. 
A  modified  type  of  the  non-synchron- 
ous gap  is  shown  in  Fig.  114  where  ten 
stationary  sparking  points  are  mounted  on 
two  brass  sectors.  A  light  contact  arm, 
A,  B,  carrying  two  spark  discharge  points 
is  attached  to  the  shaft  of  a  high  speed 

mi-         j  r    .1  •        i      .          .        Fig      114 — Another     Type     of    Non-synchronous     Dis- 

motor.      The  advantage  of  this  design   is  charger. 


Fig.   113 — Non-synchronous  Rotary  Discharger. 


ROTOR 


STATIONARY 
ELECTRONS 


MOTOR 


APPLIANCES   FOR  A   RADIO   TRANSMITTER.  103 

that  the  spark  discharge  shifts  rapidly  from  one  set  of  stationary  electrodes  to  the  other 
thereby  preventing  the  discharge  points  from  overheating.  Other  modified  types  of  this 
discharger  are  in  use  but  they  operate  on  the  same  fundamental  principle. 

The  principal  advantage  of  the  non-synchronous  discharger  is  that  it  produces  a 
musical  spark  tone  from  a  source  of  low  frequency  alternating  current.  As  will  be  seen 
from  the  diagram  of  Fig.  115,  although  the  condenser  is  discharged  at  a  uniform  rate,  suc- 
cessive discharges  are  not  of  constant  amplitude.  In  Fig.  115  the  successive  maximum 
amplitudes  of  the  charging 'current  are  indicated  at  points  a,  b,  c,  d,  e,  f,  g,  etc.,  and  the 

instant   during   the   cycle  at  which   the 

A  C  J[  £  ^  spark  discharges  by  the  mark  X.     It  is 

easily  seen  that  the  spark  frequency  is 
very  much  greater  than  the  frequency 
of  the  charging  current,  but  the  suc- 
cessive spark  discharges  are  not  of 
constant  power;  but  since  they  are 
timed  uniformly,  a  semi-musical  note 
composed  of  fundamental  tones  coupled 
with  a  number  of  overtones  is  pro- 
Fig.  115 — Showing  the  Spark  Discharge  is  Superposed  duced,  having  a  pitch  far  more  pleasing 
on  Charging  Current.  to  thfi  eaf  than  that  obtained  by  the 

plain  discharger.  By  very  careful  adjustment  of  the  speed  of  the  rotor,  a  non-synchronous 
gap  can  be  made  to  discharge  the  condenser  at  points  of  equal  amplitude  during  successive 
alternations  of  the  charging  current.  This  results  in  a  more  uniform  spark  note  than  that 
obtained  by  ordinary  adjustments. 

(c)  For  several  well  defined  reasons  the  synchronous  rotary  spark  discharger 
has  found  almost  universal  favor.  Contrary  to  the  non-synchronous  type,  the  disc 
is  mounted  on  the  shaft  of  the  alternating  current  generator  to  permit  the  dis- 
charge of  the  condenser  to  be  accurately  timed  with  the  alternations  of  the  charg- 
ing current. 

The  disc  of  the  synchronous  discharger  may  be  either  of  metal  or  of  some 
insulating  material  that  will  withstand  the  peripheral  strain  but  for  mechanical 
reasons,  the  metal  disc  is  preferred.  This  is  firmly  keyed  to  the  generator  shaft 
but  not  necessarily  insulated*  therefrom.  The  disc  always  will  have  discharge 
electrodes  of  the  same  number  as  the  field  poles  of  the  generator,  and  if  the  sta- 
tionary electrodes  are  arranged  so  that  they  can  be  shifted  through  a  small  arc, 
then  by  careful  adjustment  a  high  pitched  "screeching"  spark  note  will  be  secured. 
A  note  of  this  characteristic  is  particularly  desirable  for  copying  signals  through 
the  interference  of  static  discharges  at  the  receiver;  particularly  if  the  frequency 
of  the  charging  current  is  500  cycles  per  second. 

In  event  that  the  frequency  of  the  charging  current  is  500  cycles,  the  condenser 
will  be  charged  and  discharged  1,000  times  per  second,  and  the  resultant  spark 
note  will  have  a  musical  pitch  corresponding  thereto. 

The  construction  of  the  disc  discharger  supplied  with  the  2  K.  W.  500  cycle  sets  of  the 
Marconi  Company  is  shown  in  Fig.  116,  where  a  heavy  steel  disc  carrying  30  copper  spark 
electrodes  is  firmly  keyed  to  the  motor  generator  shaft.  The  disc  is  enclosed  in  a  steel 
muffling  drum  which  also  acts  as  a  support  for  the  two  stationary  electrodes  A,  B,  which 
are  thoroughly  insulted  therefrom.  In  addition,  this  drum  is  designed  so  that  it  can  be 
shifted  through  an  arc  of  approximately  25°  by  the  adjusting  rod  R.  This  adjustment 
permits  the  most  favorable  sparking  point  to  be  located  and  results  in  synchronous  dis- 
charges, giving  a  clear,  musical  spark  note. 

Not  only  must  the  correct  position  of  the  stationary  electrodes  be  found  for 
successful  working,  but  they  must  be  adjusted  to  give  the  shortest  possible  dis- 
charge gap  without  the  electrodes  actually  touching.  In  the  Marconi  gap,  the 
stationary  electrodes  can  be  lowered  .003  of  an  inch  at  a  time,  and  the  electrodes 
locked  in  position  to  prevent  actual  contact  with  the  revolving  disc. 

The  muffling  drum  is  thoroughly  ventilated  by  a  set  of  air  scuppers  cast  on  the 
disc  which  supply  a  uniform  blast  of  air  for  cooling  the  quenched  gap  of  the 
Marconi  panel  sets  as  well. 

*The  disc  of  the  rotary  gap,  in  the  2  K.  W.  500  cycle  set  of  the  Marconi  Company  is  connected 
to  earth. 


104 


PRACTICAL   WIRELESS   TELEGRAPHY. 


The  necessity  for  mounting  the  disc  discharger  on  the  shaft  of  the  alternator  will  be 
at  once  apparent  for  if  the  disc  were  mounted  on  an  independent  motor,  any  variation  in  the 
line  voltage  would  cause  the  "coming  together  of  the  electrodes"  and  the  alternations  of 
the  charging  current  to  fall  out  of  step,  but  when  the  disc  is  mounted  on  the  shaft  any 
reduction  of  the  frequency  of  the  charging  current  will  be  immediately  compensated  for 
by  the  simultaneous  reduction  of  the  speed  of  the  disc. 

In  the  diagram  of  Fig.  116,  the  moving  electrodes  (mounted  on  the  disc)  are  shown  at 
1,  2,  3,  4,  etc.  The  spark,  for  example,  passes  from  stationary  electrode  A  to  an  electrode 
on  the  disc,  through  the  disc  and  out  at  electrode  B.  The  effective  length  of  the  rotary 
discharge  gap  is  much  greater  than  the  actual  distance  between  the  stationary  and  revolving 
electrodes  would  indicate;  actually  the  spark  discharge  begins  long  before  the  electrodes 


STATIONARY/ 
ELECTRODES 


MUFFLING 
DRUM. 


Fig.  116 — 2  K.  W.  Synchronous  Disc  Discharger  of  the  American  Marconi  Company. 


are  directly  opposite.  Hence  for  favorable  working  the  minimum  distance  between  the 
electrodes  should  be  about  .005  inch.  This  will  give  a  clear  spark  discharge  and  will  not 
subject  the  condenser  to  abnormal  strain. 

One  great  advantage  of  the  synchronous  discharger  is  that  it  permits  the 
handling  of  very  large  powers.  In  fact,  the  Marconi  Company  have  constructed 
and  successfully  operated  such  dischargers  at  500  K.  W.  with  marked  results. 

An  additional  advantage  of  this  gap  is  that  it  prevents  the  oscillations  in  the 
aerial  circuit  being  retransferred  to  the  closed  circuit  resulting  in  more  efficient 
radiation;  in  other  words  the  quenching  is  reliable. 

The  one-half  kilowatt  sets  of  the  American  Marconi  Company  are  fitted  with 


APPLIANCES    FOR    A    RADIO    TRANSMITTER.  105 

synchronous  dischargers  which  are  practically  a  duplicate  of  the  larger  size  gaps 
such  as  furnished  with  the  2  K.  W.  sets. 

(d)  The  construction  of  the  quenched  spark  discharger  is  shown  in  Fig.  117 
where  a  number  of  heavy  copper  plates  separated  by  fiber  insulating  washers 
micanite,  or  other  insulating  material,  are  placed  in  an  iron  rack  and  compressed 
by  means  of  a  pressure  bolt.  The  thickness  of  the  wrashers  is  chosen  so  that  the 
space  between  the  sparking  surfaces  of  a  single  set  of  plates  does  not  exceed  .01 
inch. 

A  groove  is  cut  in  each  plate  over  which  the  inside  edge  of  the  washer  rests. 
This  prevents  the  spark  discharging  at  the  very  edge  of  the  washer  which  would 


Fig.   117 — Modern  Quenched  Spark  Discharger  with  Motor  Blower. 

soon  cause  a  short  circuit.  The  insulating  washers  are  specially  treated  so  that 
the  discharge  surface  is  airtight.  This  not  only  aids  in  quenching  out  the  primary 
oscillations,  but  gives  a  noiseless  discharge  as  well. 

It  will  be  noted  from  the  photograph,  that  the  gap  is  cooled  by  a  small  motor-driven 
blower  mounted  on  the  base  which  forces  a  draught  of  air  through  the  base  of  the  rack 
against  the  cooling  flanges  of  the  plates. 

Since  the  quenched  discharger  had  been  found  to  be  effective  with  comparatively  low 
voltages,  the  potential  of  the  transformer  secondary  rarely  exceeds  15,000  volts,  pressures 
as  low  as  6,000  volts  being  frequently  employed.  Approximately  1,200  volts  are  required 
for  each  gap  and  the  number  that  will  be  used  in  any  case  depends  upon  the  voltage  of  the 
transformer. 

The  great  advantage  of  the  quenched  gap  lies  in  its  ability  to  prevent  the  energy  of 
the  oscillations  induced  in  the  aerial  circuit  being  retransferred  to  the  condenser  circuit, 
even  when  the  oscillation  transformer  is  closely  coupled.  Therefore  the  antenna  oscillates 


106 


PRACTICAL   WIRELESS   TELEGRAPHY. 


at  single  fundamental  frequency  and  will  radiate  a  single  wave.  The  ideal  spark  gap 
is  one  which  permits  the  closed  circuit  to  oscillate  for  a  period  of  just  sufficient  length  to 
build  up  the  oscillations  in  the  aerial  circuit  to  their  maximum  amplitude.  Then  the 
oscillations  in  the  condenser  circuit  should  cease -,  permitting  the  antenna  to  continue  in 
oscillation  until  its  energy  is  dissipated. 

If  the  coupling  between  the  open  and  closed  circuits  is  sufficiently  loose,  quenching  of 
the  primary  oscillations  will  be  obtained  with  practically  any  type  of  gap  but  the  advantage 
of  this  particular  type  lies  in  the  fact  that  quenching  is  secured  with  very  close  coupling, 
which  gives  increased  values  of  antenna  current.  The  precise  action  taking  place  in  a 
transmitter  employing  a  quenching  gap  can  be  shown  as  in  Fig.  118  where  a  group  of 
oscillations  in  the  open  and  closed  circuits  of  a  transmitter  appear.  It  will  be  seen  that 
the  closed  circuit  oscillates  for  a  very  brief  period,  or  until  the  oscillations  in  the  aerial 
circuit  reach  their  maximum  amplitude  after  which  the  oscillations  in  the  closed  circuit 
will  be  "quenched"  out. 

Summarizing  the  foregoing,  it  will  be  seen  that  the  quenched  gap  offers  sev- 
eral advantages,  if  used  in  properly  designed  and  tuned  circuits : 

(1)  It  is  noiseless  in  operation; 

(2)  Has  no  moving  parts; 

(3)  Permits  the  use  of  low  voltage  transformers; 

(4)  Gives  synchronous  discharges  (when  properly  adjusted); 

(5)  Gives  very  large  values  of  antenna  current  because   of  closer  coupling. 
The  quenched  discharger  has  not  been  found  satisfactory  for  very  high  powers 

and  in  such  installations  some  form  of  the  Marconi  disc  discharger  is  employed. 

102.  Adjustment   of   the    Spark   Note. — It   is   highly   important   that    the 

spark  gap  of  any  radio  transmitter  be  ad- 
justed to  give  a  uniform  discharge,  having, 
if  possible,  a  musical  pitch.  A  uniform 
spark  discharge  not  only  permits  the  sig- 
nals to  be  more  easily  deciphered  at  the  re- 
ceiving station  through  atmospheric  or 
static  discharges,  but  also  permits  better 
formation  of  the  characters  of  the  tele- 
graph code  on  the  part  of  the  sending  oper- 
ator. In  addition,  the  pitch  of  the  note  has 
a  marked  effect  on  the  response  of  the  re- 
ceiver telephone  diaphragm,  there  being  a 
critical  spark  frequency  for  maximum  re- 
sponse with  different  types  of  telephones. 
In  the  case  of  the  plain  spark  discharger, 
the  pitch  of  the  note  while  governed 
primarily  by  the  frequency  of  the  alter- 
nator, is  also  dependent  upon  other  factors 
such  as, 

(1)  The  length  of  the  gap; 

(2)  The  voltage  of  the  generator; 

(3)  The  capacity  of  the  condenser; 

(4)  The  design  of  the  spark  electrodes; 

(5)  The  nature  of  the  surrounding  medium 
whether  gas  or  air. 

Now  the  capacity  of  the  condenser  in 
the  usual  radio  set  is  a  fixed  quantity, 
hence,  the  correct  note  for  this  gap  is  found 
by  increasing  or  decreasing  the  length  of 
the  discharge  gap  or  by  adjustment  of  the 
transformer  voltage. 

Blunt  discharge  electrodes  give  an  ir- 
regular spark  note  whereas  sharp  pointed 
electrodes  give  a  high  pitched  note,  but  they 
generally  diminish  the  amplitude  of  the 


CLOSED 


Fig.  118 — Showing  the  Oscillations  in  Closed  and  Open 
Circuits    of   a   Properly    Adjusted   Transmitter. 


oscillation.     A  compromise  between  all  these  conditions  is  usually  effected. 

The  correct  spark  tone  for  the  rotary  gap  is  obtained  by  adjusting  the  position  of  the 


APPLIANCES   FOR   A   RADIO   TRANSMITTER. 


107 


stationary  electrodes  until  synchronous  discharges  are  obtained,  and  also  by  regulating  the 
length  of  the  gap  and  the  voltage  of  the  transformer. 

The  note  of  the  quenched  gap  is  adjusted  by  variation  of  the  number  of  gaps  or  by  regu- 
lation of  the  generator  voltage.  By  proper  adjustment,  synchronous  discharges  are  readily 
obtained  as  will  be  evidenced  by  the  uniformity  of  the  note.  (See  Appendix,  Section  G.) 

103.  Oscillation  Transformers. — The  functions  performed  by  the  oscilla- 
tion transformer  of  a  radio-transmitter  may  be  summed  up  as  follows : 

(1)  Transfers  energy  from  the  closed  circuit  to  the  open  circuit; 

(2)  Permits  the  wave  length  of  either  circuit  to  be  increased  or  decreased  (by 

variation  of  the  self  inductance  of  either  coil); 

(3)  Permits  some  adjustment  of  the  damping  of  the  oscillations  flowing  in  the 

aerial  system. 

The  degree  of  coupling  between  the  primary  and  secondary  circuits  of  an 
oscillation  transformer  can  be  altered  in  three  ways: 

(1)  By  drawing  the  windings  apart  mechanically; 

(2)  By  variation  of  the  self  inductance  of  either  winding; 

(3)  By  turning  one  winding  at  a  right  angle  to  the  other. 

All  three  methods  are  in  use.    The  oscillation  transformer  used  in  the  2  K.  W.  240 


,\  SEC,/ 
\  \  \    /  /  / 


Fig.  120— Showing  the  Effect  of  Turn- 
ing Secondary  at  Right  Angles 
to  Primary. 

cycle  sets  and  the  60  cycle  sets  of  the 
Marconi  Company  is  shown  in  Fig. 
119.  The  primary  winding  has  eight 
turns  of  copper  tubing  approximately 
15  inches  in  diameter,  spaced  1  inch 
apart,  the  tubing  being  about  %  inch 
in  diameter.  The  adjacent  turns  of 
the  primary  winding  are  well  insu- 
lated by  means  of  specially  designed 
porcelain  insulators.  The  secondary  is 
made  up  of  heavy  stranded  insulated 
copper  cable  wound  on  a  hard  rubber 
spool.  Since  the  inductance  of  the  secondary  is  fixed,  variation  of  the  fre- 
quency of  the  oscillations  is  secured  by  adjustment  of  the  aerial  tuning  inductance. 
The  secondary  winding  is  mounted  so  that  it  can  be  turned  at  a  complete  right 


Fig.     119 — Type    A    Oscillation    Transformer    of    the 
American  Marconi  Company. 


108 


PRACTICAL   WIRELESS   TELEGRAPHY. 


angle  to  the  primary  or  at  any  required  intermediate  position,  which  permits  close 
regulation  of  the  transformer  coupling.  The  effect  of  tilting  the  secondary  at  a 
right  angle  to  the  primary  is  shown  in  Fig.  120  where  the  coupling  is  practically 
zero.  It  will  be  seen  that  the  lines  of  force  of  the  primary  are  parallel  with  the 


PRIM 


COPPER  STRIP, 


VARIABLE 
CONTACT 


Fig.     121 — Showing    Position     of 
Maximum  Coupling  of  Type 
A   Oscillation   Trans- 
former. 


Fig.   122 — Pancake  Type  Oscillation  Transformer. 


turns  of  the  secondary  and  therefore  no  induc- 
tion takes  place  in  the  secondary  but  in  the  posi- 
tion, Fig.  121,  the  lines  of  force  cut  the  secondary 
turns  at  a  right  angle  and  the  coupling  is  maxi- 
mum. 

In  all  transmitting  sets  it  is  advantage- 
ous to  ha\e  the  inductance  of  the  primary 
or  the  secondary  winding  continuously  va- 
riable, i.  e.,  the  construction  must  be  such  that 
the  entire  inductance  can  be  gone  over  inch  by 
inch  with  a  sliding  contact.  An  oscillation  trans- 
former of  this  type  is  shown  in  Fig.  122.  The  primary  and  secondary  windings 
consist  of  a  spiral  of  copper  ribbon  placed  edgewise  on  an  insulating  disc.  The 
handle  is  fitted  with  a  sliding  contact  which  permits  any  amount  of  inductance  to 
be  inserted  in  the  circuit,  inch  by  inch.  The  same  handle  that  varies  the  induct- 
ance is  used  to  draw  the  primary  and 
the  secondary  windings  apart,  thus 
varying  the  coupling. 

In  a  certain  foreign  radio  system,  an 
inductance  element  known  as  a  variometer 
inductance  is  employed  to  change  the  de- 
gree of  coupling  between  the  closed  and 
open  circuits  of  radio-frequency.  Briefly, 
it  consists  of  the  windings  A  and  B,  Fig. 
123,  comprising  two  coils  which  can  be 
magnetically  opposed  to  each  other.  The 
coils  are  connected  in  series  and  when  di- 
rectly opposite  the  inductance  is  at  a  mini- 
mum value,  but  when  B  is  drawn  away 
from  A  the  inductance  gradually  increases, 
maximum  inductance  being  obtained  when 
they  are  drawn  completely  apart. 

If  the  variometer  is  connected  In  the 
aerial  system  as  in  Fig.  124,  and  its  self- 
inductance  gradually  increased,  the  mutual 
induction  between  the  open  and  closed 
circuits  will  be  increased  and  therefore  the  FiS-  123— Showing  Fundamental  Principle  of  the  Vario- 
coupling. 

To  be  efficient  and  to  have  a  minimum  resistance,  variometers  must  be  wound  with 
stranded  copper  cable  made  of  a  great  number  of  small  strands  each  of  which  is  insulated 
from  neighboring  ones.  A  stranded  cable  made  in  this  way  is  called  "Litzendraht"  which 


APPLIANCES   FOR   A   RADIO   TRANSMITTER. 


109 


affords  a  conductor  of  maximum  con- 
ductivity for  high  frequency  currents. 

Oscillation  transformers  may  have 
fixed  values  of  inductance  for  both  the 
primary  and  secondary  windings  as  in 
Fig.  125.  In  one  type,  the  primary  has 
a  single  turn  of  copper  cable  wound 
around  6  or  8  turns  of  smaller  cable, 
constituting  the  secondary  winding. 
The  coupling  is  rather  loose — about  8 
per  cent,  for  the  average  aerial.  The 
necessary  changes  of  wave  length  are 
obtained  by  variation  of  the  capacity  of 
the  condenser  in  the  closed  circuit  and 
the  inductance  of  the  aerial  circuit. 


104.  Aerial  Tuning  Inductance. 

—The  aerial  inductance  coil  may  be 
of  the   continuously  variable  type 
having    a    sliding    contact    which 
bears  on  an  edgewise  copper  strip 
as  in   Fig.   126,  or  of  the  round  barrel  type  made  of  stranded  copper  cable 


Fig.    124— Showing   How   Coupling  is   Changed  by   Vario- 
meter. 


COPPER  STRIP 


Fig.    125 — Square  Coil  Type   Oscillation  Transformer. 

as  in  Fig.  127.  The  inductance  of 
the  latter  type  is  altered  by  means 
of  a  plug  contact.  It  is,  of  course, 
essential  that  adjacent  turns  be  well 
insulated  from  one  another  and 
separated  so  that  there  will  be  no 
sparking  between  them.  Also  the 
coil  must  be  mounted  on  an  insu- 
lated base  or  on  specially  devised 
insulators  to  prevent  the  high  volt- 
age current  leaking  to  nearby  con- 
ductors, or  conducting  material. 

105.  The  Short  Wave  Con- 
denser.— One  form  of  this  con- 
denser as  constructed  by  the  Mar- 
coni Company,  has  four  glass  plates 
15  by  15  inches  covered  with  tinfoil 

12  by    12   inches.      Each   plate   has   a       Fig.  126 — Continuously  Variable  Aerial  Tuning  Inductance. 


110 


PRACTICAL   WIRELESS   TELEGRAPHY. 


capacity  of  approximately  .002  mfds.  and  since  four  are  connected  in  series,  the 

resulting  value  is  .0005  mfds. 

A  very  recent  type  of  short  wave  condenser  manufactured  by  the  Marconi  Company  is 

shown  in  Fig.  128.  Four  copper  plated 
glass  jars  are  mounted  on  a  metal  rack 
and  connected  together  as  follows:  Con- 
nection is  made  from  the  binding  post  E 
of  the  left  hand  inside  jar  A  to  the  aerial 
system.  The  outer  coating  of  jar  A  rests 
on  the  inside  coating  of  the  jar  B.  The 
outside  coating  of  jar  B  makes  contact 
with  the  outside  coating  of  the  jar  D 
through  the  metal  rack.  The  circuit  con- 
tinues on  from  that  point  in  just  the  re- 
verse direction,  to  that  cited  in  the  first 
case,  finally  ending  at  the  binding  post  of 
jar  C.  The  four  jars  are  thus  connected 
in  series  and  have  combined  capacity  of 


- 


Fig.   127 — Drum  Type  Aerial  Tuning  Inductance. 


.0005  mfds.     The  short  wave  condenser  may  be  connected  in  series  with  either  the  aerial 
wires  or  the  earth  lead. 

106.  High  Potential  Condensers. — The  Leyden  jar  type  of  condenser  has 
been  described  in  paragraph  84.  The  assembly  of  the  flat  plate  condenser  is 
shown  in  Fig.  129  where  three  banks  of  condenser  plates  are  connected  in  series. 
The  student  should  note  from  this  diagram  how  the  connecting  tabs  are  brought 
out  from  the  sheets  of  tinfoil  and  the  manner  in  which  the  plates  are  stacked  up. 
It  will  be  seen  from  the  detail  that  a 
left  hand  tab  plate  is  stacked  up 
against  a  right  hand  tab  plate  and  so 
on  until  the  required  capacity  is  ob- 
tained. When  one  group  is  com- 
plete, the  entire  unit  is  bound  with 
canvas  tape  and  immersed  in  a  tank 
of  oil. 

The  tinfoil  is  first  attached  to  the 
condenser  plates  by  means  of  a  good 
grade  of  fish  glue  after  which  it  is  al- 
lowed to  dry.  The  plate  is  then  coated 
with  shellac  or  hot  paraffin  and  after  dry- 
ing again  is  ready  for  use. 

Owing  to  the  general  inconvenience 
of  assembling,  an  oil  condenser  of  this 
type  and  the  bother  of  replacing  a  broken 
plate,  in  case  of  breakdown,  it  is  grad- 
ually being  eliminated  from  ship  sets.  In 
case  of  accident,  the  copper  plated  Ley- 


Fig.    128 — Short    Wave    Condenser    American    Marconi 
Company. 


den  jar  affords  a  distinct  advantage  be- 
cause a  broken  jar  can  be  replaced  within 
a  few  minutes.  In  event  the  charging  current  exceeds  15,000  volts  a  series  parallel  connec- 
tion of  condenser  plates  is  required.  This  connection  divides  the  voltage  between  the  two 
banks  and  thereby  reduces  the  strain  on  the  dielectric. 

Compressed  air  condensers  are  in  use  but  the  expense  of  construction  does  not  warrant 
their  general  adoption.  Briefly,  the  construction  is  as  follows :  A  number  of  interleaved 
steel  plates  are  enclosed  in  a  cylindrical  tank,  one  set  of  plates  being  insulated  from  the 
structure.  The  tank  is  pumped  to  pressure  of  250  pounds  air  pressure  which  has  sufficient 
dielectric  strength  to  withstand  potentials  of  25,000  volts.  The  dielectric  losses  in  a  con- 
denser of  this  type  are,  of  course,  zero  and  a  permanent  rupture  of  the  dielectric  medium 
is  not  possible.  The  tanks  are  very  cumbersome  and  some  difficulty  is  experienced  in 
keeping  them  air  tight. 

107.  High    Frequency    "Choking"    Coils.  —During  the  charge  and  discharge 


APPLIANCES   FOR   A   RADIO   TRANSMITTER. 
30,000  VOLTS 


111 


Fig.    129— Assembly   of  the   Oil  Plate   Condenser. 

of  the  condenser  of  the  closed  circuit,  a  back  pressure  is  exerted  on  the  secondary  winding 
of  the  high  voltage  transformer.  Although  the  impedance  of  the  spark  gap  circuit  is  so  low 
compared  to  that  of  the  secondary  winding  of  the  transformer  that  the  spark  leaps  the  gap 
in  preference,  the  discharge  of  the  condenser  may  cause  sufficient  difference  of  potential 

between  the  turns  of  the 
secondary  units  of  the 
transformer  to  puncture  the 
insulation  and  short  circuit 
adjacent  layers.  Possible 
punctures  from  this  source 
may  be  obviated  by  means 
of  "choking"  coils  of  rela- 
tively high  self  inductance 
connected  between  the 
transformer  and  the  con- 
denser as  shown  at  N1  and 
N2,  Fig.  130.  These  coils 
consist  of  a  few  turns  of  fine 
wire  wound  in  the  form  of 
a  spiral,  or  in  a  single  layer 
on  a  porcelain,  glass  or  hard 
rubber  tube.  They  offer  but 
little  impedance  to  the  low 
frequency  current  flowing 
from  the  secondary  winding 
Fig.  i29a— Marconi  Glass  Plate  Condenser  of  the  transformer,  but  they 


112 


PRACTICAL   WIRELESS   TELEGRAPHY. 


greatly   impede   the   radio-frequency  oscillations.    Hence  the  secondary  winding  is  protected 
from  injury. 

108.  High  Voltage  Transformers. — The  construction  of  the  open  and 
closed  core  types  of  transformers  of  the  Marconi  Company  is  shown  in  Figs.  131 
and  132. 

The  primary  coil  of  the  open  core  transformer  is  wound  on  a  core  consisting  of 
a  bundle  of  fine  iron  wires  bound  in  circular  form.  The  core  is  covered  with 


PRIM 


CONDEN5EH 


.  Fig.  130 — High  Frequency  Choking  Coils. 

several  layers  of  insulating  tape  followed  by  two  or  three  layers  of  rather  coarse 
copper  wire,  such  as  No.  10  or  No.  12  S.  C.  C.  The  entire  primary  is  then  in- 
serted in  an  insulating  tube.  The  secondary  winding  is  composed  of  several  sec- 
tions, each  consisting  of  a  number  of  thin  pancakes  wound  with  rather  fine  wire, 
such  as  No.  26  to  No.  32. 

Dry  insulation  is  employed  and  in  the  larger  sizes  a  blast  of  air  is  blown- 
through  the  primary  core  to  keep  the  transformer  cool.  The  open  core  transform- 
ers are  designed  for  primary 
potentials  from  110  to  500 
volts  and  for  frequencies  from 
60  to  500  cycles.  The  sec- 
ondary potential  rarely  ex- 
ceeds 15,000  volts. 

The  transformer  shown  in 
Fig.  131  is  one  of  the  open- 
core  type  supplied  with  the 
2  K.  W.  transmitters  of  the 
American  Marconi  Company. 
The  primary  core  and  wind- 
ing are  inserted  in  a  Micarta 
insulating  tube,  but  the  sec- 
ondary turns  are  split  into 
groups,  each  consisting  of 
several  small  pancakes  of  fine 
wire  connected  in  series.  The 
secondary  turns  are  covered 
by  an  insulating  tube  which 
is  thoroughly  clamped  in 
place.  A  safety  spark  gap 
mounted  on  the  top  of  the 
transformer  case  has  a  third 
electrode,  which  is  connected 
to  earth.  The  primary  termi- 
nals are  underneath  the  base 
and  the  secondary  terminals 

Fig.   131—2   K.   W.   500  Cycle  Open   Core  Transformer.  are  mounted  On  the  top. 


APPLIANCES   FOR   A    RADIO    TRANSMITTER. 


113 


Fig.  132  shows  the  construction  of  the  closed  core  transformers.  The  core 
is  rectangular  and  made  up  of  thin  sheets  of  iron  which  are  insulated  from  one 
another.  The  primary  and  secondary  windings  are  mounted  one  over  the  other 
on  the  middle  leg.  The  entire  transformer  is  immersed  in  a  semi-liquid  grease, 
which  softens  at  about  115  degrees  Fahrenheit. 

The  closed  core  trans- 
former is  now  supplied  with 
all  Marconi  sets.  It  has  the 
advantage  of  efficiency,  and 
cheapness  of  construction, 
and  requires  but  a  small 
amount  of  space  for  erec- 
tion. 

A  safety  gap  is  provided 
to  protect  the  secondary 
winding  in  case,  the  rotary 
gap  is  thrown  out  of 
synchronism  with  the 
charging  current.  In  event 
of  such  happening,  the 
spark  will  discharge  across 
the  gap  and  also  to  earth 
through  a  third  electrode 
(the  metallic  cone-shaped 
projection  on  the  case). 
The  transformer  case  is 
connected  to  earth.  If  the 
transformer  is  to  be  taken 
apart  to  replace  the  wind- 
ings, in  cold  climates,  the 


CASE. 


Fig.  132 — Showing  Construction  of  2  K.  W.  Closed 
Core  Transformer. 

case  must  be  heated  to  melt  the  grease. 


It  is  important  that  some  part  of  the  low  frequency  circuit  of  a  radio-transmitter  have  a 
certain  amount  of  magnetic  leakage  when  the  transmitting  key  is  pressed,  for  it  will  be 
self-evident  from  the  foregoing  diagrams  that  when  the  spark  discharges  across  the  spark 
gap,  the  secondary  winding  of  the  transformer  is  practically  on  short  circuit.  The  ordinary 
closed  core  transformer  will  under  these  conditions  draw  an  excess  of  current  with  conse- 
quent danger  of  burning  it  out.  If  the 
transformer  is  provided  with  a  magnetic 
leakage  gap  such  as  the  air  gap  A  in  the 
transformer  core,  Fig.  133,  the  lines  of 
force  generated  by  the  secondary  current 
will  take  the  path  across  this  gap  and  in 
consequence  the  self-inductance  of  the 
primary  winding  will  remain  practically 
constant.  Even  on  direct  short  circuit  of 
the  secondary,  the  transformer  fitted  with 


1 

"\  \ 

1 

44 

?        ( 

PRIM 

/ 

P 

I™1* 

(        UJ 

1               -4- 

A           | 

44- 
*£ 

•> 

\ 

T-* 

\ 

tl" 

> 

1  - 

'                   1 

\ 

N* 

^_ 

Fig.    133 — Closed     Core     Transformer     with     Magnetic 
Leakage  Gap. 


magnetic   leakage   gap   will   not   burn   out, 

and  hence  is  particularly  suitable  for  the 

excitation  of  radio-transmitters.      In  the   Marconi  sets,  the  requisite  amount  of  magnetic 

leakage  takes  place  at  the  generator,  the  armature  being  constructed  to  have  very  high  values 

of  self-inductance. 

The  2  K.  W.  500  cycle  transformers  furnished  with  the  Marconi  panel  trans- 
formers require  no  external  reactance  coil  except  when  the  set  transmits  at  the 
wave  length  of  300  metres.  To  reduce  the  power  for  the  reduced  condenser 
capacity  at  this  wave  length,  a  small  fixed  reactance  coil  is  connected  in  series 
with  the  primary  winding. 


114 


PRACTICAL   WIRELESS   TELEGRAPHY. 


WINDING 


If  the  primary  power  is  to  be  reduced  by  steps  a  reactance  regulator  is  required 
with  any  type  of  set. 

109.  Reactance  Regulators. — The  flow  of  current  through  an  alternating 
current  transformer  is  preferably 
regulated  by  a  "choking"  coil  of 
high  self-inductance  rather  than  by 
a  variable  resistance.  These  coils, 
which  are  termed  choking  coils  or 
reactance  regulators,  generally  con- 
sist of  a  few  turns  of  heavy  insu- 
lated copper  wire  wound  over  an 
iron  core. 

Two  types  are  shown  in  Figs.  134  and 
135.  When  an  alternating  current 
passes  through  the  coil,  the  correspond- 
ing magnetic  lines  of  force  rise  and  fall 
with  the  current,  but  the  collapse  of  the 
field  sets  up  a  counter  E.  M.  F.  which 
acts  as  an  effective  resistance.  The  flow 


Fig.    134— Primary   Reactance   Coil. 


of  current  can  be  varied  by  a  multi- 
point switch  as  in  Fig.  134,  or  by  draw- 
ing the  iron  core  in  and  out  of  the  windings  as  in  Fig.  135.  In  the  design  of  Fig.  135  a  "U" 
shaped  iron  core  is  inserted  in  two  windings  connected  in  series.  The  self-induction  of  the 
coil  is  varied  by  moving  the  core  in  and  out  the  windings. 

Certain  types  of  high  potential  transformers  are  designed  for  direct  connection  to  the 
power  mains,  and  therefore  they  do  not  require  a  reactance  regulator. 

110.  Aerial  Changeover  Switch. — The  object  of  the  aerial  switch  is  to 
connect  the  aerial  alternately  from  the  transmitting  to  the  receiving  apparatus. 
Because  of  the  potentials  of  the  transmitting  apparatus,  the  receiving  apparatus 

must  be  completely  disconnected  from 
the  transmitting  aerial  by  a  switch 
with  a  break  of  at  least  6  inches  in 
length. 

An  aerial  switch  of  recent  design,  the 
type  S.  H.  switch  of  the  Marconi  Company 
is  indicated  in  Fig.  136  with  the  necessary 
diagram  of  connections  for  a  commercial 
radio  installation.  When  the  handle  of  the 
switch  is  thrown  upward,  the  apparatus  is 
connected  for  receiving,  but  in  the  down- 
ward position  the  transmitting  apparatus 
functions.  The  switch  controls  several 
F,g.  135-  J  Shaped  Reactance  Coil.  other  important  drcuits  as  well  as  the 

aerial  circuit  connection.    Thus  when  the  switch  is  thrown  to  the  transmitting  position, 

(1)  Contacts  A,  B,  close  the  D.  C.  circuit  to  the  generator  field; 

(2)  Contacts  C,  D,  close  the  circuit  to  the  automatic  motor  starter  or  to  a  motor 

driven  blower; 

(3)  Contacts  E,  F,  close  the  circuit  to  the  primary  winding  of  the  power  trans- 

former ; 

(4)  The  spring  contacts   R,  R,  spring  together  and  connect  the  transmitting 

apparatus  to  the  aerial  system. 

(5)  The  circuit  to  the  primary  winding  of  the  receiving  tuner  is  broken  between 

G  and  K; 

(6)  Contacts  L  and  M  place  the  detector  on  short  circuit; 

(7)  Contacts  N  and  O  short  circuit  the  telephones. 

The  reverse  connections  and  disconnections,  of  course,  take  place  when  the  switch   is 
thrown  to  the  receiving  position. 


APPLIANCES   FOR  A   RADIO   TRANSMITTER. 


115 


TUNER 


Fig.    136— Connections   of  a   Modern   Aerial   Changeover   Switch. 

It  will  be  evident  from  the  foregoing  that  during  the  sending  period  at  a  given  station,  the 
receiving  tuner  is  thoroughly  disconnected  from  the  antenna  and  during  the  receiving 
period  the  circuit  to  the  field  winding  of  the  generator  is  interrupted  because  otherwise 
the  current  flowing  to  the  meters  will  induce  interfering  currents  in  the  receiver  which 
may  swamp  out  the  radio  signals  ^  from  a  far  distant  station.  Other  types  of  aerial 
change-over  switches  are  described  in  Part  IX. 

111.  Transmitting  Keys.— The  sig- 
nalling keys  for  radio  telegraphy  are 
of  heavier  construction  than  those  used 
in  landline  telegraphy  because  they  are 
required  to  interrupt  large  values  of  cur- 
rent. It  is  found  possible  to  break  cur- 
rent up  to  35  amperes  by  a  hand 
manipulated  key,  but  for  current  in  ex- 
cess of-this  value,  a  magnetically  operated 
relay  type  of  key  is  employed. 

One  type  manufactured  by  the  Mar- 
coni Company  appears  in  Fig.  137. 

Fig.137~A  Hand  Manipu.a.ed  Te.egraph  Key.  ^^    4^  ^ad    platinum    pofnts     for 

breaking  the  current,  but  silver  contacts  of  increased  area  have  proven  a  satis- 
factory substitute. 

The  circuit  of  an  electromagnetic 
key  is  shown  in  Fig.  138.  A  pair  of 
magnets  M,  M1  are  wound  to  approxi- 
mately 150  ohms  resistance  and  con- 
nected to  110  volts  D.  C.  through  a 
small  W.  U.  sending  key.  Extra  large 
contacts  A,  B  are  mounted  on  the  arma- 
ture and  stationary  posts  which  may 
break  50  or  60  amperes  without  arcing. 
Another  type  has  a  solenoid  winding 
with  a  plunger  extending  into  a  box  of 
oil.  The  contacts  make  and  break  the 
circuit  under  oil,  reducing  arcing  to  a 
minimum. 

In  certain  high  power  stations  spe- 
cially constructed  electromagnetic  keys 

interrupt  the  circuit  from  the  secondary  Fi*   138~  circuits  of  Ma*net5c 

winding  of  the  transformer  instead  of  the  primary.     500  kilowatts  can  be  broken  in  this 
manner. 


1 10  VOLT  D.C. 


PART  VIII. 


AERIALS  OR  ANTENNAE 

112.  FUNCTION  OF  THE  AERIAL.  113.  DETERMINATION  OF  THE 
WAVE  LENGTH  FROM  THE  DIMENSIONS  OF  AN  AERIAL. 
114.  FUNDAMENTAL  CONSIDERATIONS.  115.  VARIOUS  TYPES  OF 
AERIALS.  116.  DIRECTIONAL  AERIALS.  117.  STANDARD  MAR- 
CONI AERIAL.  118.  THE  DECK  INSULATOR.  119.  INSTALLA- 
TION OF  THE  AERIAL.  120.  EARTH  CONNECTION.  121.  RADIA- 
TION. 122.  ANTENNA  DECREMENT.  123.  TRANSMISSION 
RANGE. 

112.  Function  of  the  Aerial. — The  function  of  the  aerial  or  antennae  of  any 
given  radio  station  is  two-fold: 

(1)  To     radiate  energy  in  the  form  of  electromagnetic  waves; 

(2)  To    absorb  part  of  the  energy  of  the  waves  sent  out  by  the  transmitter. 

The  stations  employing  two  aerials,  one  for  transmitting  and  one  for  re- 
ceiving are  few  in  number;  in  fact,  the  great  majority  of  wireless  stations  employ 
a  single  antenna  for  both  purposes,  which  in  its  simplest  form  consists  of  a  wire 
of  silicon  bronze,  copper  or  aluminum  supported  vertically  by  an  insulator  from 
a  mast  or  tower.  The  aerials  for  present  day  ship  use  consist  of  2,  3  or  4  wires 
connected  in  parallel  with  adjacent  conductors  placed  from  2l/2  to  3l/2  feet  apart. 

Not  all  aerials,  however,  are  supported  vertically;  the  great  majority  have  a 
flat  top  portion  which  extends  horizontally  to  a  distance  varying  from  40  to  6,000 
feet,  the  length  depending  upon  the  use  to  which  the  aerial  is  to  be  put  or  to  the 
power  of  the  station. 

Experimentation  and  reasoning  show  that  the  various  types  of  aerials  do  not 
radiate  with  equal  intensity  in  all  directions.  A  certain  type,  for  instance,  has  a 
distinctly  directive  characteristic  and  will  radiate  its  energy  more  strongly  in  a 
given  direction.  Still  another  type  confines  the  greater  part  of  its  radiation  to 
two  opposite  directions.  Not  only  does  the  factor  of  radiation  require  to  be 
taken  into  account  but  added  to  this  are  the  tuning  qualities  of  the  radiated  wave, 
which  are  somewhat  affected  by  the  design  of  the  aerial.  It  is,  therefore,  well  to 
review  at  once  the  factors  governing  the  type  of  aerial  adopted  in  any  particular 
case. 

Chief  among  the  controlling  factors  are : 

(1)  The  space  available  for  erection; 

(2)  The  total  expense  of  installation; 

(3)  The  radiating  properties; 

(4)  The  desired  characteristic  of  the  radiated  wave  (whether  "sharp,"  "broad" 
or  "directional"). 

Without  regard  to  the  foregoing  considerations,  the  dimensions  of  an  aerial 
are  governed : 

(1)  By  the  length  of  the  wave  to  be  radiated; 

(2)  By  the  space  available  for  erection. 

As  we  have  shown,  an  aerial  possesses  both  distributed  capacity  and  induct- 
ance which  combined  give  it  a  defined  period  of  oscillation  when  a  charge  of 
electricity  is  applied  to  it.  These  oscillations  will  set  up  a  wave  motion  the 
length  of  which  is  related  to  the  capacity  and  inductance  of  the  system  in  the 


AERIALS   OR   ANTENNAE. 


117 


following  manner:  If  the  capacity  (C)  be  measured  in  farads  and  the  inductance 
(I.)  in  henries,  the  length  of  the  wave  will  be  equal  to  4  X  V  X  V  LC,  where 
V  =  the  velocity  of  electric  waves  in  ether  (300,000,000  meters  per  second). 

Now  the  length  of  the  radiated  wave  can  be  increased  by  connecting  a  coil  of 
wire  (aerial  timing  inductance)  at  the  base  of  the  aerial,  or  decreased  by  con- 
necting a  condenser  in  series  at  the  base,  but  there  are  certain  limitations  in  either 
direction  as  will  be  presently  explained.  Moreover  with  concentrated  inductance 
at  the  base  of  the  aerial  we  can  no  longer  use  the  simple  formula  above  for  de- 
termining the  length  of  the  wave  which  must  now  be  modified  to  read,  wave 

27T  

length  -       -  XV  X  V  LC,  where  K  is  a  certain  correction  factor  the  ratio  of 

K 
the  inductance  of  the  coil  to  the  total  inductance  of  the  aerial. 

Experiment  indicates  that  it  is  not  advisable  to  load  an  aerial  with  inductance  to  radiate  a 
wave  more  than  four  times  the  natural  wave  length,  because  the  insertion  of  greater  amounts 

will  reduce  the  flow 
of  current  considerably 
and  thereby  reduce  the 
range  of  the  trans- 
mitter. In  fact,  the 
maximum  transmitting 
range  is  obtained  in 
every  case,  when  the 
aerial  radiates  at  a 
wave  length  near  to  its 
fundamental  wave.  The 
use  of  localized  induc- 
tance decreases  its  de- 
crement, but  does  not 
add  to  the  energy  of  the 
oscillations.  Hence, 
large  amounts  of  load- 
ing inductance  are,  wherever  possible,  to  be  avoided.  The  addition  of  inductance  in  the 
antenna  circuit  up  to  a  certain  point  is  favorable  to  the  tuning  qualities  of  the  radiated  wave, 
but  beyond  this  point,  unless  the  capacity  of  the  aerial  be  increased,  the  flow  of  current  will 
be  reduced  considerably. 

A  small  amount  of  inductance  is,  of  course,  inserted  at  the  base  of  the  aerial  to  act  as 
the  secondary  winding  of  the  oscillation  transformer,  but,  generally,  it  need  not  exceed 
10,000  to  15,000  centimeters.  In  all  cases,  the  length  of  an  aerial  is  governed  principally 
by  the  length  of  the  wave  to  be  radiated,  but  to  permit  the  insertion  of  the  secondary 
inductance,  the  dimensions  should  be  such  that  its  natural  wave  length  will  be  less  than 
the  length  of  the  radiated  wave. 

It  is  easily  seen  that  if  the  length  of  the  aerial  be  increased,  both  the  inductance 
and  capacity  will  be  increased  and  the  radiated  wave  accordingly.  Hence,  if  the 
aerial  is  found  after  erection  to  be  too  long  for  the  required  wave,  either  the 
length  of  the  aerial  can  be  reduced  or  the  length  of  the  wave  can  be  artificially 
reduced  by  means  of  a  series  condenser.  Increased  flow  of  antenna  current  will  be 
obtained  in  any  case  if  an  aerial  is  selected,  the  dimensions  of  which  will  permit 
the  required  wave  to  be  obtained  without  the  series  condenser,  but  aboard  ship  an 
aerial  of  the  correct  dimensions  cannot  always  be  obtained ;  in  fact,  a  short  wave 
condenser  is  generally  used  for  the  300  meter  wave  and  a  small  amount  of  induct- 
ance for  the  600  meter  wave. 

113.  Determination  of  the  Wave  Length  from  the  Dimensions  of  an  Aerial. 

— The  fundamental  or  natural  wave  length  of  an  aerial  can  be  computed  directly  from  the 
dimensions,  the  most  notable  contributions  to  the  determination  having  been  made  by  Prof. 
G.  W.  Howe*  and  Dr.  L.  Cohen;  but  generally  these  formulae  are  too  complicated  for  the 
practical  worker;  hence,  the  following  approximate  method  may  be  used.  First,  it  may  be 

*cTw.   Howe,   Wireless  World,   Dec.,    1914,   Jan.,    1915. 


Fig.   139 — Vertical  or  Fan  Aerial. 


118 


PRACTICAL   WIRELESS- TELEGRAPHY. 


INSULATORS 


Fig.   140— Umbrella  Type  Aerial. 


mentioned  that  the  natural  wave  length  of  a  four-wire  horizontal  aerial  with  the  wires  spaced 
about  2l/z  feet  apart  will  be  approximately  4.4  to  4.8  times  the  total  length  of  the  aerial,  that  is, 

the  length  from  the  extreme  end  down  to 
the  apparatus  at  the  station  house.  This 
factor  is,  of  course,  extremely  approxi- 
mate, for  it  does  not  take  into  account  the 
presence  of  nearby  conductors,  such  as 
chimneys,  metal  roofs,  mast  guys,  trees, 
etc.,  some  of  which  have  the  effect  of  in- 
creasing the  capacity  of  the  system. 

For  an  aerial  system  comprising,  let  us 
say,  four  wires  spaced  2^2  feet  apart,  the 
natural  wave  length  is  not  much  greater 
than  that  of  a  two-wire  aerial  with  equiva- 
lent spacing,  because  although  the  addition 
of  wires  increases  the  capacity  slightly,  it 
also  decreases  the  total  inductance  of  the 
system,  and  generally  these  two  factors 
nearly  offset  each  other.  Thus,  in  a  given 
instance,  a  two-wire  aerial  with  wires 
spaced  8  feet  apart  had  a  natural  wave 
length  of  325  meters,  and  the  addition  of 
two  wires  (inserted  between)  merely  in- 
creased the  wave  length  to  345  meters. 

If    the 'wires    of   an    aerial    are    widely 

spaced  apart,  the  capacity  is  greatly  increased,  and  the  total  inductance  somewhat  increased, 
due  to  the  diminishing  of  the  mutual  inductance  between  adjacent  wires.  But  the  increase 
of  capacity  exceeds  the  decrease  of  inductance,  and  the  natural  wave  length  will  therefore 
be  increased  considerably.  In  practice,  it  is  the  custom  to  space  the  wires  so  they  will  be 
mutually  inductive,  the  distance  between  adjacent  wires  not  exceeding  three  feet. 

114.  Fundamental  Considerations. — In  the  design  of  a  wireless  aerial  the 
important  points  to  be  considered  are: 

(1)  The   aerial   wires   must   possess   great   tensile   strength   and   be   of   good 
conductivity; 

(2)  There  should  be  several  parallel  wires; 

(3)  Adjacent  wires  should  be  spaced  2  to  3  feet; 

(4)  The  wires  must  be  thoroughly  insulated  at  all  points  of  support; 

(5)  If  possible,  the  aerial  should  be  erected  in  a  clear  space  and  at  least  a 
half-wave  length  from  all  metallic  structures. 

The  matter  of  antenna  insulation  is  extremely  important ;  the  antenna  insula- 
tors should  not  only  possess  high  specific  resistance,  but  they  should  also  be  of 
considerable  length  to  prevent  the  high  voltage  current  discharging  over  them  to 
some  nearby  metallic  conductor.  Particular  care  must  be  taken  to  insulate  the  free 
end  of  an  aerial  because,  owing  to  the  fact  that  the  voltage  and  current  are  not 
uniformly  distributed  in  vertical  aerials,  a  very  much  greater  potential  exists  at 
the  top  than  at  the  base.  This  non-uniform  distribution  exists  to  some  extent  in 
all  types  of  aerials,  but  is  less  manifest  in  the  flat  top  aerials  than  in  vertical  types. 
Hence  any  conductors  at  the  top  or  the  free  end  of  the  aerial  that  would  tend  to 
start  leakage  of  the  high  voltage  current  should  be  widely  separated  and  thor- 
oughly insulated  from  the  aerial  wires. 

115.  Various  Types  of  Aerials. — Four  general  types  of  aerials  are  in  use: 

(1)  The    vertical  or  fan  aerial; 

(2)  The  umbrella  aerial; 

(3)  The  inverted  "L"  flat  top  aerial; 

(4)  The  "T"  aerial. 

(a).  The  vertical  cwrial  shown  in  Fig.  139  consists  of  a  fan  or  harp  of  copper 
or  silicon  bronze  wires  held  vertically  into  space  by  a  wooden  mast,  a  steel  tower 
or  any  convenient  structure  of  sufficient  height.  The  wires  of  the  harp  may  or 
may  not  be  joined  at  the  top.  All  wires,  however,  must  converge  at  the  lower  end 


AERIALS   OR   ANTENNAE.  119 

where  they  enter  the  station  house  and  are  connected  to  the  apparatus.  The  free 
end  of  the  vertical  aerial  must  be  well  insulated  to  prevent  direct  leakage  to  the 
supporting  halyards  or  stays,  and  at  the  lower  end  the  wires  must  be  stayed  to 
take  the  strain  off  the  station  house  roof  insulator. 

Although  the  ver- 
tical aerial  is  ac- 
knowledged to  be 
the  best  radiator 
of  electromagnetic 
waves  practically 
equal  results  can  be 
obtained  from  a  flat 
top  aerial  (of 
increased  d  i  m  e  n  - 
sions)  with  a  less 
expensive  supporting 


Fig.   141—  Inverted  "L"  Flat  Top  Aerial.  Structure. 

tive  of  the  degree  of 

efficiency  obtained,  a  vertical  aerial  could  not  possibly  be  used  aboard  vessels  on 
account  of  the  derrick  booms,  mast  guys  and  smoke  funnels  which  take  up  the 
space  that  would  be  required  for  erection: 

(b).  The  umbrella  aerial  shown  in  Fig.  140  receives  its  name  from  its  general 
shape  and  similarity  to  an  umbrella.  It  will  be  noted  in  this  diagram  that  a  number 
of  wires  spread  radially  in  several  directions  from  a  common  center  at  the  top  of 
the  mast,  and  that  a  wire  extends  therefrom  to  the  apparatus  in  the  station  house. 
The  ribs  of  the  umbrella  generally  are  about  two-thirds  the  length  of  the  mast,  but 
the  guying  out  wires  must  be  six  to  seven  times  the  length  of  the  vertical  part. 

Although  it  has  been  found  to  be  of  some  value  for  portable  military  sets,  the 
umbrella  aerial  is  scarcely  employed  in  commercial  working  except  in  certain  high 
power  stations  designed  and  erected  by  German  engineers. 

(c).  The  inverted  "L"  flat  top  aerial  of  Fig.  141  is  almost  universally  em- 
ployed for  ship  service.  It  consists  of  a  number  of  parallel  wires  stretched  between 
two  masts  and  attached  on  either  end  to  wooden  or  metal  spreaders,  which  are 
thoroughly  insulated  from  the  supporting  halyards.  The  horizontal  wires  A  to  B 
are  called  the  "flat  top  portion"  and  the  vertical  wires,  the  "lead-ins." 

The  lead-in  wires,  which  should  have  equal  conductivity  to  the  wires  in  the 
flat  top,  are  attached  to  one  end  of  the  horizontal  wires,  then  passed  through  a  deck 
insulator  and  finally  connected  to  the  apparatus  within  the  station.  The  flat  tops 
of  the  ships  under  control  of  the  Marconi  Wireless  Telegraph  Company  of  America 
vary  from  75  to  250  feet  in  length  and  the  lead-ins  from  70  to  150  feet. 

If  the  lead-ins  are  attached  to  the  center  of  the  flat  top,  as  in  Fig.  142,  the  aerial 
is  said  to  be  of  the  "T"  type. 

Either  "T"  or  "L"  aerials  are  almost  universally  employed  on  vessels,  princi- 
pally because  they  are  more  convenient  to  install.  Those  in  the  Marconi  service 
have  2,  4  or  6  wires  ;  the  majority,  however,  have  four  wires  spaced  from  2^2 
to  3y2  feet. 

The  fundamental  wave  length  of  the  "T"  aerial  is  invariably  less  than  that  of 
the  inverted  "L"  type  of  the  same  dimensions.  When  the  lead-ins  of  a  given  aerial 
are  removed  from  the  end  of  the  flat  top  and  attached  to  the  center,  the  total  in- 
ductance will  be  less  than  with  the  previous  connection,  and  since  by  this  change, 
the  capacity  of  the  system  remains  practically  unchanged,  the  length  of  the  radiated 
wave  will  be  less  than  in  the  case  of  the  "L"  aerial.  This  is  easily  understood  if 
viewed  in  the  following  manner  :  Beginning  at  the  point  where  the  lead-ins  are  at- 
tached, the  two  ends  of  the  flat  top  may  be  considered  as  two  aerials  in  parallel, 
and  as  is  well  known,  the  inductance  of  two  parallel  conductors  is  less  than  that 


120 


PRACTICAL   WIRELESS   TELEGRAPHY. 


a 


Fig.    142— -T"   Flat  Top   Aerial. 


of  either  taken  separately;  therefore,  the  total  inductance  of  the  antenna  will  be 
reduced  and  the  radiated  wave  accordingly. 

To  illustrate  the  point:  A  four-wire  inverted  L  type  of  aerial,  100  feet  in  length,  60  feet 
in  height,  has  capacity  of  .0004  microfarads,  and  inductance  of  62,000  centimeters.  A  "T" 
aerial  of  the  same  dimensions  has  capacity  of  .0004  microfarads,  and  inductance  of  37,000 
centimeters.  Keeping  in  mind  the  simple  formula  (X  =  38  x  VLC)  for  computing  the  wave 
length  of  a  radiative  oscillator,  the  change  in  wave  length  brought  about  by  this  connection 
is  easily  determined.  The  wave  length  of  the  "L"  aerial  will  be  approximately  188  meters 
and  of  the  "T"  aerial,  approximately  145  meters. 

Since  the  standard  transmitters  of  the  Marconi  Company  are  designed  for  the 
use  of  the  300,  450  and  600  meter  wave,  the  precaution  must  be  taken  to  select 
an  aerial  of  such  dimensions  that  a  moderate  degree  of  efficiency  will  be  obtained 

on  the  two  shorter 
waves  and  the  maxi- 
mum degree  of  effi- 
ciency at  the  longer 
wave;  that  is,  we 
must  provide  an  aer- 
ial that  will  give  the 
highest  possible  aer- 
ial current  for  each 
of  the  three  standard 
waves.  An  aerial 
having  a  fundamen- 
tal wave  length  ap- 
proximating 325  me- 
ters gives  good  values  of  antenna  current  at  the  450  meter  and  600  meter  adjust- 
ment, and  a  fair  value  at  the  300  meter  wave  adjustment,  but  an  aerial  of  correct 
dimensions  for  this  fundamental  wave  length  obviously  cannot  always  be  erected. 

On  certain  large  vessels,  the  distance 
between  masts  is  so  great  that  if  the 
wires  were  suspended  the  total  length 
between  them,  a  series  condenser  would 
be  required  for  the  600  meter  wave.  Be- 
cause the  wave  length  of  an  aerial  can 
never  be  reduced  by  a  series  condenser 
to  less  than  one-half  its  natural  length, 
the  300  meter  wave  could  not  be  obtained 
in  a  case  of  this  kind,  and  it  would  there- 
fore become  necessary  to  cut  off  a  por- 
tion of  the  horizontal  wires,  to  keep  the 
radiated  wave  within  limits.  It  is  usual 
in  such  cases  to  stretch  the  wires  from 
mast  to  mast  and  insert  an  insulator 
about  50  to  100  feet  from  one  end  or  at 
such  distance  as  will  permit  the  required 
length  of  wave  to  be  obtained.  On  the 
other  hand,  the  aerials  of  small  vessels, 
such  as  tugboats,  etc.,  have  to  be  loaded 
with  large  amounts  of  inductance  to  ob- 
tain the  600  meter  wave.  On  such  ves- 
sels the  aerial  is  frequently  made  up  of 
eight  wires  in  order  to  obtain  the  maxi- 
mum possible  capacity. 

With  few  exceptions,  all  ships'  aerials 
in   the   Marconi   Service   require   a   short  F»&-  143— Bellini-Tosi  Directional  Aerial, 

wave  series  condenser  for  the  300  meter 

wave,  but  should  the  aerial  be  found  upon  measurement  of  its  wave  length  to  require  a 
series  condenser  for  the  450  meter  wave,  either  the  length  of  the  flat  top  would  be  reduced 


5PARK  GM> 


Height  of 

No.  of 

Natural  wave 

Capacity 

flat  top. 

wires. 

lerg'.h. 

mfds. 

96ft. 

6 

374 

.00128 

125 

6 

368 

.00145 

87 

4 

355 

.00115 

95 

4 

412 

.0015 

90 

6 

360 

.0015 

100 

6 

325 

.00132 

92 

4 

285 

.00075 

150 

4 

426 

.00096 

90 

6 

360 

.0023 

55 

6 

230 

.00085 

110 

6 

290 

.0009 

98 

6 

425 

.0024 

85 

4 

380 

.00082 

AERIALS    OR   ANTENNAE.  121 

or  the  lead-ins  be  attached  to  the  center.  The  general  rule  adopted  by  the  American  Mar- 
coni Company  is  as  follows :  If  the  length  of  the  flat  top  is  less  than  125  feet,  the  inverted 
L  aerial  is  employed,  but  if  it  exceeds  125  feet,  the  lead-ins  are  attached  to  the  center.  This 
permits  the  use  of  the  three  standard  waves,  gives  good  values  of  antenna  current  and 
affords  a  very  favorable  decrement  of  the  oscillations. 

As  a  matter  of  general  information  we  may  cite  examples  of  actual  measurements  of 
the  wave  length  of  commercial  ships'  aerials.  This  data  appears  in  the  following  table: 

Length  of 

Type.  flat  top. 

L  208  ft. 

T  200 

L  150 

T-Imp.  250 

L  200 

L  120 

T  130 

T  250 

L  200 

L  125 

T  151 

L  200 

L  170 

We  see  from  this  table  that  the  capacity  of  some  aerials  is  rather  large,  but 
generally  this  increase  is  due  to  the  presence  of  nearby  metallic  structures  of  some 
sort,  such  as  mast  guys,  derrick  booms,  or  steel  masts. 

We  may  sum  up  the  advantages  or  disadvantages  of  the  various  types  of  aerials  briefly, 
as  follows :  The  vertical  aerial  is  a  very  efficient  radiator  of  electric  waves ;  but  if  large 
amounts  of  energy  are  to  be  radiated,  a  large  aerial  and  a  very  high  supporting  structure 
are  required ;  the  cost  of  the  latter  may  be  prohibitive.  The  umbrella  aerial  does  not  quite 
possess  the  radiating  properties  of  the  vertical  aerial  and  due  to  the  fact  that  the  ribs  of 
the  umbrella  must  be  guyed  out  several  hundred  feet  from  the  base  of  the  supporting 
mast,  a  large  area  is  required  for  its  erection.  But  with  this  disadvantage,  the  umbrella 
aerial  is  found  to  be  of  some  value  in  portable  military  stations,  where  the  ribs  of  the 
aerial  act  as  guy  supports  as  well  as  radiators  of  the  electric  waves. 

The  "L"  and  "T"  aerials  also  do  not  radiate  quite  as  efficiently  as  the  simple  vertical  aerial 
of  the  same  dimensions,  but  they  have  slightly  lower  njS^ral  decrements,  and  therefore,  radiate 
a  sharp  wave  with  less  localized  inductance.  Flat  top  aerials  can  be  erected  at  much  less 
expense  than  vertical  aerials  of  much  less  length  and  they  prove  to  be  just  as  effective 
at  less  initial  expense.  Over  all,  it  must  be  kept  in  mind  that  irrespective  of  the  degree 
of  efficiency  obtained,  the  flat  top  aerial  only  can  be  conveniently  employed  aboard  ship. 

Wave  Distortion. — Better  signals  are  frequently  obtained  from  a  given 
transmitter  by  the  use  of  a  receiver  aerial,  a  part  of  which  is  horizontal  such  as  the  "L" 
and  "T"  types.  This  may  be  accounted  for  in  the  following  way:  It  has  been  assumed 
by  several  investigators  that  the  lower  end  of  the  loops  of  force  or  ether  strain  radiated 
by  an  aerial  are  retarded  in  their  propagation  over  dry  earth,  e.  g.,  the  lower  part  travels 
more  slowly  than  the  upper  part,  which  causes  the  field  of  the  radiated  wave,  so  to  speak, 
ta  be  tilted  against  the  direction  of  movement. 

In  any  receiver  aerial  the  greatest  amount  of  energy  is  taken  from  the  passing  wave 
when  the  wires  are  at  right  angles  to  the  magnetic  field  and  parallel  to  the  static  field. 
It  necessarily  follows  that  if  the  loops  of  force  radiated  from  the  aerial  of  Fig.  99  do  not 
hold  their  vertical  position  but  are  tilted  forward,  better  distances  would  be  covered,  in 
fact,  more  energy  would  be  induced  in  the  receiver  aerial,  if  part  of  it  were  vertical  and 
part  horizontal. 

116.  Directional  Aerials. — It  has  been  definitely  proven  by  Marconi  that  a 
horizontal  aerial  in  which  the  length  of  the  fiat  top  largely  exceeds  the  height  will  radiate 
more  strongly  in  the  direction  opposite  to  the  free  end.  Also  the  strongest  signals  will  be 
obtained  at  the  receiver  by  an  aerial,  the  free  end  of  which  points  in  the  direction  opposite 
to  the  free  end  of  the  transmitting  aerial. 

Taking  full  advantage  of  the  unsymmetrical  radiation  from  such  aerials,  the  high  power 
transoceanic  stations  of  the  Marconi  Company  employ  directional  aerials  exclusively.  The 


PRACTICAL  WIRELESS  TELEGRAPHY. 


AERIALS   OR  ANTENNAE. 


123 


flat  tops  of  such  aerials  are  from  3,000  to  6,000  feet  in  length,  and  from  300  to  450  in 
height.    The  fundamental  wave  length  is  close  to  10,000  meters. 

The  greater  part  of  the  energy  radiated  from  a  transmitter  may  be  confined  to  a  given 
direction  by  the  use  of  the  Bellini-Tosi  aerial  shown  in  Fig.  143.  A  triangular  aerial, 
A,  B,  C,  is  supported  by  a  vertical  mast.  The  two  sides  of  the  rectangle  A,  C.  and  B,  C. 
make  an  angle  of  30°  with  the  vertical,  and  the  third  side  is  horizontal  to  the  earth  with 
the  coil  L-l  inserted  at  the  center.  L-l  is  inductively  coupled  to  a  spark  transmitter  and 
tuned  to  syntony.  An  aerial  of  this  type  will  radiate  its  energy  with  greatest  intensity  in 
the  direction  of  its  own  plane,  the  radiation  in  a  direction  perpendicular  to  the  triangle 
being  zero.  In  any  direction  making  an  angle  0  with  the  plane  of  the  triangle,  the  intensity 
of  the  radiation  varies  as  the  cosine  of  the  angle  0.  The  triangular  aerial  may  be  used 
for  receiving  purposes  as  well,  and  if  the  aerial  be  arranged  so  that  it  can  be  turned  on 
its  axis  the  greatest  strength  of  signal  will  be  obtained  when  the  plane  of  the  antenna 
points  in  the  direction  of  the  wave  front.  In  modified  form  these  aerials  are  made  use  of 
in  connection  with  the  Marconi  direction  finder,  which  will  be  described  in  Part  XIII. 

117.  Standard  Marconi  Aerial. — Now  that  the  fundamental  design  of  the 
various  aerials  has  been  shown,  the  construction  of  a  standard  Marconi  ship's 
aerial  (American  Marconi  Company)  will  be  described  in  detail.    It  will  be  noted 
from  the  diagram  of  Fig.  144  that  the  flat  top  contains  six  silicon  bronze  wires,  each 
containing  7  strands,  No.  18  wire.    The  wires  are  preferably  equally  spaced  and 
are  attached  to  spruce  spreaders  from  14  to  18  feet  in  length  which  in  turn  are 
attached  to  the  running  halyards  by  a  bridle  which  is  made  up  of  strop  insulators. 
These  insulators  consist  of  ^j  inch  Russian  boat  rope  which  is  partly  covered  by  a 
hard  rubber  tube.     The  space  between  the  tube  and  the  rope  is  filled  with  hot 
sulphur  which  when  cold,  hardens  and  keeps  out  moisture.    Both  ends  of  the  bridle 
terminate  in 'a  heart-shaped  shackle  to  which  a  galvanized  steel  halyard  wire  for 
raising  and  lowering  the  aerial  is  attached. 

To  prevent  the  spreaders  from  swaying,  side  stays  are  attached  to  the  ends  (of 
the  spreader)  and  fastened  to  the  mast.  Twenty- four-inch  hard  rubber  rod  insu- 
lators are  inserted  in 
the  rope  to  prevent 
leakage  of  the  cur- 
rent in  wet  weather. 
It  will  be  seen  also 
that  each  wire  of  the 
aerial  is  insulated  by 
a  2-foot  hard  rubber 
rod  which  is  at- 
tached to  the  spread- 
er by  an  eyebolt. 

The  lead-in  wires 
are  attached  to  one 
end  of  the  flat  top 
and  fastened  to  the 
deck  or  cabin  insu- 
lator. To  remove 
the  strain  from  this 
insulator,  two  hard 

rubber  rod  insulators  are  attached  to  the  aerial  and  fastened  to  the  deck  by  a  wood 
screw  as  shown  in  the  drawing.  Positive  connection  is  made  between  the  lead-ins 
and  the  flat  top  by  means  of  a  Mclntyre  connector  shown  in  the  detail  of  Fig.  144A. 
The  general  design  shown  in  Fig.  144  is  not  always  duplicated  in  detail,  but 
wherever  possible  it  is  adhered  to. 

118.  The  Deck  Insulator. — The  transmitting  aerial  must  be  well  insulated 
at  the  point  where  it  enters  the  radio  operating  cabin  by  an  insulator  that  will  with- 
stand at  least  30,000  volts.    One  form  of  deck  insulator  is  shown  in  Fig.  145.    A 


FLAT  TOP  WIRE. 


THESE   ENDS  SHULO 
BE  AT  LEAST  10"  LONG 


MAC  INTYRE  SLEEVES. 
DOUBLE  TUBE  fOR 
*7- 18  WIRE  SOLDERED 
NOT  TWISTED 


Fig.  144a — Mclntyre  Sleeve  Connector. 


124 


PRACTICAL   WIRELESS    TELEGRAPHY. 


CrO 

Fig.    145— Bradfield    Type    Deck 
Insulator. 


long  hard  rubber  tube  T  about  2  inches  in  diameter  has 
a  brass  rod  extending  through  it  which  terminates  at 
each  end  in  a  wire  connecting  lug,  W .  The  tube  is 
threaded  at  the  center  to  take  two  wooden  blocks  A,  B, 
one  of  which  is  placed  above  the  deck,  the  other  under- 
neath. After  these  blocks  are  drawn  up  tightly,  the 
wood  screws  are  inserted.  To  insure  a  watertight  joint, 
a  piece  of  canvas  strip  is  placed  underneath  the  blocks 
and  thoroughly  covered  with  white  lead.  A  metal 
hood,  H,  fastened  to  the  exposed  end  of  the  tube  pro- 
tects it  from  dampness. 

The  latest  type  of  deck  insulator  is  shown  in  Fig. 
146,  wherein  a  large  electrose  insulator  has  a  heavy 
brass  rod  moulded  securely  into  it  which  terminates  at 
either  end  in  a  connecting  lug.  The  outside  of  the  insu- 
lator is  threaded,  and  after  it  is  inserted  in  the  hole  in 
the  deck,  it  is  drawn  up  tight  by  the  collar,  R,  which  is 
threaded  on  the  inside.  Other  types  of  deck  insulators 
are  in  use,  but  those  just  described  are  indicative  of 
modern  practice. 

119.  Installation  of  the  Aerial. — The  aerial  of  a 
ship  should  be  installed,  if  possible,  in  such  a  way  that 
the  lead-ins  will  be  free  and  clear  of  all  mast  guys  or 
derrick  booms.     The  further  removed  they  are  from 
parallel  conductors  of  any  kind,  the  less  will  be  the 
danger  of  induced  currents. 
In  measuring  off  the  length  of  the  flat  top  wires,  approximately  ten   feet  must  be  al- 
lowed at  each  end  for  the  bridle,   the  insulators  and  the  reef  block;   hence  20  feet  must 
be  subtracted  from  the  distance  between  masts 
at  the  start.    The  distance  between  masts  can 
generally   be   obtained    from   the   ships'   plans 
or  by  actual  measurement  from  mast  to  mast 
on  the  deck.     Each  wire  is  accurately  meas- 
ured between   two  points  marked  off  on  the 
deck   and  the   wire   cut  and   attached   to   the 
insulators.     Six   or  eight  inches  must  be  al- 
lowed for  serving  the  wire  through  the  eye 
of  the  antenna  insulator. 

All  connections  are  thoroughly  made  on 
the  deck,  after  which  the  halyards  are  passed 
through  the  reef  block  and  fastened  to  the 
shackle  on  the  bridle.  The  aerial  is  then  pulled 
into  space,  it  being  freed  from  all  obstructions 
by  three  or  more  assistants. 

120.  Earth  Connection. — The  con- 
nection from  the  transmitting  apparatus 
to  the  earth  plate  should  be  direct  as  pos- 
sible and  the  conductor  should  be  one  of 
high  conductivity.  In  marine  installa- 
tions the  earth  wire  is  simply  fastened  to 
the  metal  bulkhead  by  a  bolt,  the  earth 
connection  being  thus  made  through  the 
hull.  Usually  this  connection  is  not  more 
than  two  or  three  feet  in  length. 

On  wooden  vessels  connection  is  made 
to  the  propeller  shaft  in  the  engine  room  Fig.  i46— Electrose  Moulded  insulator. 


BRASS  ROD 


LUG 


UBBER 


AERIALS   OR   ANTENNAE.  125 

or  to  the  water  drip  at  the  smoke  funnels.  Installations  grounded  or  earthed  in 
this  manner  generally  do  not  transmit  as  far  as  those  on  vessels  with  steel  hulls. 
Occasionally  a  wooden  vessel  is  placed  in  drydock  and  200  or  300  feet  of  copper  or 
yellow  metal  nailed  on  the  bottom  for  an  earth  capacity.  A  strip  of  copper  is  then 
led  from  it  to  the  radio  cabin. 

The  earth  plate  for  the  land  station  is  sometimes  very  elaborate  and  may  consist  of  a 
great  number  of  copper  or  zinc  plates  buried  in  moist  earth  to  a  depth  of  several  feet.  Tn 
addition  a  number  of  wires  are  spread  radially  from  the  station  in  all  directions,  being  laid 
particularly  directly  underneath  the  flat  top  portion  of  the  aerial.  If  the  station  is  located 
on  rock  or  dry  soil,  the  earth  wires  are  merely  laid  on  the  surface  of  the  ground  under- 
neath the  aerial.  All  wires  are  then  joined  to  a  common  terminal  and  connected  to  the 
apparatus  in  the  station. 

The  earth  plates  and  connections  at  the  Marconi  trans-atlantic  stations  are  very  elabo- 
rate and  expensive.  A  number  of  zinc  plates  are  laid  in  a  circle  of  100-foot  radius,  the 
transmitting  station  being  situated  at  the  center.  About  250  copper  cables  connect  from 
the  transmitting  apparatus  to  each  of  the  zinc  plates.  A  number  of  copper  cables  stretch 
out  radially  from  the  zinc  plates,  some  of  which  lie  directly  underneath  the  aerial.  In 
addition,  a  number  of  wires  may  be  laid  directly  underneath  the  aerial  on  the  surface  of 
the  ground. 

If  the  ground  is  particularly  damp  or  marshy,  modified  arrangements  may  be  used. 

121.  Radiation.  —  As  shown  by  Fleming  and  other  investigators,  the  power 
of  the  waves  radiated  from  an  aerial  may  be  expressed  as  follows  : 

h2 

W  =  1578  X  —  X  1L' 

\- 

where  W  =  energy    radiated   in   watts  ; 
h  =  height  of  aerial  in  meters  ; 
X  —  wave  length  of  aerial  in  meters  ; 
I  r=  current  in  amperes  at  the  base  of  the  aerial. 

For  example,  if  an  aerial  40  meters  (130  feet)  in  height,  radiates  at  the  wave  length 
of  600  meters  and  the  aerial  current  is  10  amperes,  the  energy  thrown  off  in  the  form  of 

402 

electric  waves    =  1578  X  --  X  102  =  694  watts. 
6002 

This  equation  is  based  on  the  assumption  that  the  current  is  uniformly  distributed 
throughout  the  oscillator,  which  is  not  strictly  true  in  all  types,  the  distribution  varying 
widely  according  to  the  form  of  the  antenna.  Hence,  in  order  that  the  power  of  the 
radiation  may  be  determined  with  any  degree  of  accuracy,  the  average  value  of  the  current 
must  be  determined,  and  this,  of  course,  will  be  less  than  the  current  at  the  base  of  the 
aerial. 

2          1 

The  average  value  depends  upon  the  form  of  the  antenna,  varying  from  —  to  —  times 

TT          2 

the  current  at  the  base.  If  we  denote  the  form  factor  of  the  aerial  by  the  notation  1'"  the  above 
formula  should  be  'Written  ; 

h2 
W  =  1578  -  F'  I  L> 

\- 

This  can  be  written  : 

W  -.-.  1578  X 


In  this  equation  (F  h)  is  the  effective  height  of  the  aerial  (the  form  factor  multiplied 
by  the  height)  and  in  the  case  of  the  flat  top  antenna,  the  value  of  h  is  taken  as  the 
height  of  the  vertical  portion  only;  also  the  form  factor  (F)  merely  takes  into  account 
the  average  value  of  current  in  the  vertical  part  of  the  aerial. 

The  student  will  note  that  if  the  average  value  of  current  in  the  antenna  circuit  remains 
constant,  the  amount  of  energy  radiated  from  a  given  aerial  system  depends  directly  upon 
the  effective  height.  and  inversely  upon  the  length  of  the  radiated  wave. 


126  PRACTICAL   WIRELESS   TELEGRAPHY. 

The  following  formula*  is  applicable  for  calculation  of  the  form  factor  of  any  flat  top 
aerial  oscillating  near  its  natural  period: 


/        L\          f      h     v 

F  =  0.637|  1  +  — JSin  I     I  90°* 

V        h/          Vh-r-L/ 


where  L  =  length  of  the  horizontal  part   for   inverted   L  aerials  and 

equals  one-half  total  length  flat  top  for  T  aerials, 
h  =.  height  of  vertical  portion. 

For  example,  if  we  had  an  aerial  100  feet  in  length,  60  feet  in  height,  the  value  of  the 
form    factor, 

100  v     s   60 
=  0.637 


/   100      ,   60   \ 
(  1+-   \  Sin  |  -    —  190° 
V    60  /    V  60 +  100  / 

=  0.637  X  2.6  X  Sin  (0.375  X  90° )" 
=  0.89 


Now,  if  this  aerial  is  worked  near  to  its  fundamental  wave  length,  say  320  meters,  and 
the  current  oscillating  in  the  aerial  circuit  is  6  amperes,  the  power  thrown  off  in  electric 
waves, 


=  1578X   f  °'92  x  18-4  V  X  36  =  150  +  watts. 
V      320         / 


122.  Antenna  Decrement.  —  The  damping  of  the  oscillations  flowing  in  the 
transmitting  aerial  is  principally  due  to: 

(1)  Energy  lost  by  radiation; 

(2)  Energy  lost  by  resistance  of  conductors  and  earth  plate; 

The  energy  lost  by  radiation,  is  useful  energy  because  it  goes  to  make  up  the 
wave  motion,  but  the  other  two  losses  detract  from  the  efficiency  of  the  set  and 
therefore  should  be  reduced  to  the  lowest  possible  value.  The  resistance  of  the 
aerial  conductors  can  be  reduced  to  a  minimum  by  using  a  number  of  stranded 
copper  wires  connected  in  parallel  and  in  the  average  case  the  high  frequency  re- 
sistance will  then  not  exceed  two  or  three  ohms.  The  earth  plate  resistance  is 
reduced  by  an  extended  net  work  of  conductors  preferably  buried  in  moist  earth. 

With  an  improperly  adjusted  transmitter  there  will  be  an  additional  source 
of  antenna  damping  due  to  the  re-transference  of  energy  to  the  spark  gap  cir- 
cuit. This  may  be  prevented  by  properly  adjusting  the  spark  gap  and  coupling 
at  the  oscillation  transformer. 

The  decrement  due  to  radiation  is  usually  determined  by  the  insertion  of 
localized  inductance  and  the  amount  of  inductance  required  for  a  given  radia- 
tion decrement  varies  slightly  with  the  construction,  i.  e.,  the  form  factor  of 
the  antenna.  The  antenna  is  so  designed  that  an  inductance  which  will  give 
the  correct  wave  length  will  also  give  the  desired  decrement  when  a  properly 
adjusted  transmitter  is  employed. 

Since  the  energy  radiated  in  the  form  of  electric  waves  causes  damping  of  the  oscilla- 
tions in  the  aerial  circuit,  the  loss  of  energy  by  radiation  may  be  expressed  as  an  effective 
resistance  or,  in  short,  "radiation  resistance,"  which,  of  course,  will  be  expressed  in  ohms. 
The  radiation  resistance  is  the  quantity  which  multiplied  by  the  square  of  the  average  cur- 
rent in  the  aerial  enables  us  to  determine  the  power  of  the  radiated  waves. 

The  radiation  resistance  of  a  flat  top  aerial  is  expressed, 


Where  R  =  resistance  in  ohms ; 

Fh=:  effective  height  of  aerial; 
X  =  wave  length  in  meters. 

Hence  an  aerial,  the  effective  height  of  which  is  40  meters,  operated  at  the  wave  length 
of  600  meters,  will  have  radiation  resistance  of 

*A.   S.   Blatterman,  Oct.,   1916,  issue  of  the  Wireless  Age. 


AERIALS   OR  ANTENNAE.  127 

402 

1578X =  6.9  ohms. 

6002 

We  see  from  this  formula  that  the  radiation  resistance  of  an  aerial  depends  directly 
upon  its  effective  height  and  inversely  as  the  length  of  the  radiated  wave.  It  is  also  clear 
that  increase  of  the  height  will  increase  the  wave  length  as  well,  and,  therefore,  the  increase 
of  the  power  of  the  radiation  will  not  be  so  large  with  an  increase  of  height  as  might  be 
expected  at  first  sight. 

The  radiation  resistance  (or  radiation  coefficient)  of  the  vertical  or  fan  aerials  is  gen- 
erally higher  than  that  of  the  flat  top  aerials,  but,  as  mentioned  previously,  the  flat  top 
aerials  are  less  expensive  to  erect,  more  convenient  to  install  and  since  the  radiation  resist- 
ance should  in  any  case  be  reduced  by  inductance,  they  are  more  desirable. 

We  want  the  radiation  resistance  of  an  aerial  to  be  as  high  as  possible  so  long 
as  it  does  not  seriously  damp  out  the  oscillations  and  spoil  the  tuning  qualities  of 
the  wave;  on  the  other  hand  we  want  the  frictional  resistance  (including  the  con- 
ductors and  the  earth  plate)  to  be  as  low  as  possible. 

The  oscillations  of  the  antenna  circuit  will  be  less  feebly  damped,  if  a  certain  amount 
of  inductance  is  inserted  at  the  base  but  after  a  certain  critical  value  is  reached,  the  flow 
of  antenna  current  will  be  reduced  which  will  reduce  the  power  of  the  radiated  waves. 
The  critical  value  can  be  determined  in  any  case  by  noting  the  reading  of  the  aerial 
ammeter  and  determining  the  decrement  of  the  oscillations  by  a  decremeter.  If  a  feeble 
decrement  is  obtained  at  the  expense  of  antenna  current,  the  efficiency  of  the  set  will  be 
reduced. 

If  the  effective  capacity  (C),  the  effective  inductance  (L)  and  the  effective  resistance 
(R)  of  an  aerial  are  known,  the  decrement  per  complete  oscillation  of  the  antenna  circuit 
can  be  calculated  by  the  formula  previously  given. 

s=irR|/|_ 

where  C  =  capacity  in  farads ; 

L  —  inductance  in  henries. 

Now  L  and  C  can  be  measured  by  a  wavemeter  as  will  be  shown  in  Part  11  and  the 
effective  value  of  R  can  be  determined  in  the  following  manner :  A  closed  oscillation  circuit 
containing  a  quenched  spark  discharger  is  inductively  coupled  to  the  antenna  circuit 
with  an  ammeter  connected  in  series.  At  any  particular  value  of  primary  power,  the 
reading  of  the  aerial  ammeter  is  observed.  A  closed  oscillation  circuit  having  inductance 
and  capacity  of  the  same  value  as  the  aerial  is  now  connected  across  the  secondary  winding 
of  the  oscillation  transformer  with  the  ammeter  and  a  variable  resistance  in  series.  The 
closed  circuit  is  set  into  oscillation  and  the  resistance  is  adjusted  until  the  ammeter  gives 
the  same  reading  as  when  connected  in  the  antenna  circuit.  Obviously  the  value  of  R  equals 
the  total  effective  resistance  of  the  aerial.  The  student  having  had  some  experience  in  radio 
will  recognize  this  last  circuit  as  the  usual,  "dummy  aerial"  which  is  employed  in  the 
laboratory  to  make  experimental  determinations. 

Assume  for  example  that  an  aerial  has  effective  capacity  of  .001  microfarads,  effective 
resistance  of  7  ohms  and  effective  inductance  of  100,000  centimeters,  then 

/  .000,000,001 
Decrement  =  3. 1416  X  7X  \  ~   — OOOT~~  ~-^8  per  complete  oscillation. 

A  decrement  of  this  magnitude  is  very  favorable  to  non-interference  of  radio  stations 
and  is  easily  obtained  with  properly  adjusted  spark  transmitters. 

123.  Transmission  Range — It  has  been  found  that  for  a  given  length  of 
wave  radiated  from  the  transmitter,  there  are  certain  heights  of  sending  and  receiving 
aerials,  which  will  give  the  best  signals  over  a  given  distance.  The  equation  express- 
ing the  relationship  of  these  quantities  is  as  follows: 

T  __  635  X  I*  X  hs  X  hr     .  —0.0762  X  d 

*Xd  t  V X 

Where  I8  =  current  in  sending  aerial  in  amperes ; 

Ir=  current  in  receiving  aerial  in  milliamperes ; 
hs  and  hr=  height  of  sending  and  receiving  aerial  in  feet ; 
X  —  wave  length  of  transmitter  and  receiver ; 
d  =  distance  between  in  miles. 


128 


PRACTICAL    WIRELESS   TELEGRAPHY. 


The  value  of  the  receiver  current  Ir  is  that  to  be  obtained  when  the  resistance  of  the 
receiving  aerial  is  25  ohms. 

If  the  resistance  is  greater  or  less,  the  factor  635  will  be  changed  accordingly.  The 
factor  0.0762  is  the  coefficient  of  absorption  which  denotes  the  rapidity  with  which  ether 
waves  are  absorbed  when  travelling  over  sea  water. 

If  we  assume  that  10  micro-amperes  of  current  in  the  receiving  aerial  will  create  a  just 
audible  signal  and  40  micro-amperes,  a  readable  signal,  the  required  antenna  current  at  the 
sender  for  a  given  strength  of  signal  in  the  receiver  is  readily  obtained  if  the  remaining 
factors  in  the  equation  are  known.  The  formula  is  still  the  subject  of  considerable  debate. 


Fig.  146a — 3  Kilowatt  Non-Synchronous  land  station 
type  of    transmitter   (American   Marconi  Company). 


PART  IX. 

RECEIVING  CIRCUITS,  DETECTORS  AND 
TUNING  APPARATUS 

^ 
STANDARD    MARCONI    RECEIVING    SETS. 

124.  IN  GENERAL.  125.  THE  PROBLEM.  126.  SIMPLE 
RECEIVER.  127.  THE  INDUCTIVELY  COUPLED  RECEIVER.  128. 
OTHER  METHODS  OF  COUPLING.  129.  THE  CARBORUNDUM 
DETECTOR  AND  TUNING  CIRCUITS.  130.  ADJUSTMENT  OF  THE 
INDUCTIVELY  COUPLED  TUNER.  131.  THE  ACTION  OF  THE  CAR- 
BORUNDUM CRYSTAL.  132.  ADJUSTMENT  OF  CRYSTAL  DETECT- 
ORS. 133.  DETECTOR  HOLDERS.  134.  CLASSIFICATION  OF  THE 
RECEIVING  DETECTORS.  135.  FLEMING  VALVE  DETECTOR  AND 
TUNING  CIRCUITS.  136.  MARCONI  TYPE  107-A  TUNER.  137. 
MARCONI  MAGNETIC  DETECTOR  AND  THE  MULTIPLE  TUNER 
CIRCUITS  (ENGLISH  MARCONI  COMPANY).  138.  THE  MARCONI 
TYPE  106  RECEIVING  TUNER.  139.  MARCONI  RECEIVING 
TUNER  TYPE  101  (AMERICAN  MARCONI  COMPANY).  140.  THE 
MARCONI  UNIVERSAL  RECEIVING  SET  (ENGLISH  MARCONI 
COMPANY).  141.  ELECTROLYTIC  DETECTOR.  142.  THE  THREE 
ELEMENT  VALVE  DETECTOR.  143.  A  REPEATER  VACUUM 
VALVE  CIRCUIT.  144.  THE  VACUUM  VALVE  AMPLIFIER.  145. 
AMPLIFICATION  OF  RADIO  FREQUENCIES.  146.  THE  EFFECTS 
OF  DISTRIBUTED  CAPACITY.  147.  THE  "END  TURNS"  OF  A 
RECEIVING  TUNER  AND  END  TURN  SWITCHES.  148. 
THE  VARIATION  OF  A  RADIO  FREQUENCY  INDUCTANCE. 
149.  BUZZER  EXCITATION  SYSTEMS.  150.  RECEIVING  TELE- 
PHONES. 151.  MICROPHONIC  RELAYS  OR  SOUND  INTENSIFIERS. 
152.  BROWN  AMPLIFYING  RELAY.  153.  ATMOSPHERIC  ELEC- 
TRICITY. 154.  THE  MARCONI  BALANCED  CRYSTAL  RECEIVER 
(ENGLISH  MARCONI  COMPANY).  155.  TYPE  I  AERIAL  CHANGE- 
OVER SWITCH.  156.  MARCONI  TYPE  112  RECEIVING  TUNER. 
156A.  GENERAL  ADVICE  FOR  THE  MANIPULATION  OF  A  RECEIV- 
ING TUNER. 

124.  In  General. — Told  in  brief,  the  function  of  the  receiving  aerial  is 
to  absorb  a  certain  amount  of  energy  from  the  advancing  electromagnetic  wave 
in  the  form  of  radio-frequent  oscillations;  but  more  specifically,  the  wave  motion 
induces  radio-frequent  oscillations  in  the  receiving-  aerial,  and  a  part  of  their 
energy  is  made  to  operate  some  sort  of  signal  making  instrument  within  the 
station. 

We  have  already  shown  an  electromagnetic  wave  to  be  composed  of  an  electro- 
static and  an  electromagnetic  field,  the  former  being  radiated  (in  the  case  of  the 
vertical  aerial)  perpendicular  to  the  earth  and  the  latter  at  right  angles  to  the 
transmitting  aerial  and  horizontal  to  the  earth.  It  appears  that  the  maximum 
induction  would  be  obtained  at  the  receiving  station  if  the  aerial  were  of  such 
shape  or  lay  in  such  position  that  it  would  be  at  right  angles  to  the  magnetic  field 
and  parallel  with  the  static  field.  But  if  we  take  into  account  that  the  radiated 


130 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Li 


wave  is  distorted  when  propagated  over  dry  earth,  for  example  (base  travelling 
slower  than  top),  it  is  easily  seen  that  an  aerial  having  a  part  horizontal  and  a  part 
vertical  will  receive  the  maximum  induction  therefrom. 

The  necessity  for  electrical  resonance  in  circuits  of  radio-frequency  in  which  energy 
is  transferred  by  coupling  magnetically  one  to  the  other,  has  been  explained 
in  detail.  In  fact,  for  the  maximum  induction,  it  is  just  as  essential  that 
two  open  circuit  oscillators  be  adjusted  to  substantial  resonance  as  two 
closed  circuit  oscillators.  Only  by  tuning  the  receiver  aerial  to  the  natural 
frequency  of  the  transmitter  aerial  will  the  oscillations  induced  in  the 
receiver  antennae  attain  their  maximum  amplitude;  at  all  other  adjustments 
(of  the  receiver  aerial)  the  strength  of  the  induced  current  will  be  less, 
depending  upon  the  amount  of  detuning  at  the  receiver. 

Now  the  dimensions  of  receiver  aerials  differ  widely,  and,  as 
may  be  expected,  the  circuit  of  the  receiver  antenna  contains  appli- 
ances whereby  the  oscillating  period  may  be  artificially  increased  or 
decreased  to  conform  with  the  frequency  of  the  advancing  wave 
motion.  These  tuning  devices  are  known  as : 

(1)  The  aerial  tuning  inductance; 

(2)  The  short  wave  condenser. 

By  proper  adjustment  of  these  tuning  elements  the  receiver 
aerial  can  be  electrically  tuned  to  the  wave  radiated  by  the  distant 
transmitter.  The  advancing  wave  will  then  induce  currents  at  a 
rate  tending  to  build  up  the  amplitude  of  the  receiver  current — that 
is,  each  half  wave  will  have  completed  its  work  on  the  receiver 
aerial  before  the  next  half  wave  acts.  But  under  conditions  of 
dissonance  between  the  transmitter  and  receiver  aerials,  the  react- 
ance of  the  receiving  antenna  circuit  will  not  permit  the  induced  cur- 
rent to  build  up  to  its  maximum  strength. 

The  position  occupied  in  the  antenna  circuit  by  these  tuning 
elements  is  shown  in  Fig.  147,  where  the  aerial  tuning  inductance 
is  indicated  at  L-2  and  the  short  wave  condenser  at  C.  The  third 
coil  in  this  diagram,  L-l,  is  employed  to  transfer  the  oscillations 
induced  in  the  antenna  circuit  to  a  local  detecting  circuit,  where  they 
are  translated  into  intelligible  signals.  Just  as  in  the  case  of  the  transmitter  aerial, 
the  aerial  tuning  inductance  increases  the  fundamental  period  of  oscillation  of  the 
receiver  aerial,  but  in  this  case  makes  it  responsive  to  long  waves,  while  the  short 
wave  condenser  decreases  the  fundamental  oscillating  period,  making  it  respon- 
sive to  waves  shorter  than  the  fundamental  wave  of  the  receiver  aerial. 

Starting  at  the  transmitting  apparatus,  the  entire  process  involved  in  the  induction  of 
oscillating  currents  in  a  receiver  aerial  may  be  summed  up  as  follows : 

Assume  a  transmitter  at  the  sending  station  of  the  500-cycle  synchronous  spark  type, 
giving  1,000  sparks  per  second.  Each  spark  discharge  will  induce  in  the  transmitter 
aerial  a  single  group  of  radio-frequent  oscillations,  and  each  group  will  consist  of  from 
25  to  100  complete  oscillations,  varying  in  number  as  the  decrement.  These  oscillations 
will  radiate  a  part  of  their  energy  in  the  form  of  a  wave  motion,  and  ^he  Jength  of  a 
single  wave  will  be  equal  to  their  velocity  per  second,  divided  by  their  number.  If  the 
receiver  aerial  is  adjusted  to  electrical  resonance  with  the  oscillations  of  the  transmitter, 
1,000  groups  of  radio-frequent  oscillations  will  be  induced  therein  per  second,  .  and  by 
appropriate  devices  they  can  be  made  to  operate  some  sort  of  a  signal  making  instrument, 
such  as  a  telephone  receiver. 

The  coils  L-l  and  L-2  and  the  variable  condenser  C  are  often  termed  the 
frequency  determining  elements  of  the  receiving  system,  because  they  permit  the 
natural  oscillation  frequency  of  the  receiving  aerial  to  be  changed  over  certain 
given  limits.  In  contrast  to  the  coils  of  the  transmitting  aerial  circuit,  the  induct- 
ances L-l  and  L-2  are  made  of  fine  wire  rather  than  copper  tujbing  or  heavy 
stranded  cable.  A  typical  variable  condenser  of  the  type  employed  in  the  Marconi 
wavemeter  and  receiving  tuners  is  shown  in  Fig.  147a. 


Fig.  147— Di- 
agram Showing 
Apparatus  In- 
cluded in  the 
Open  Circuit  of 
a  Receiving  Set. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS.  131 

125.  The  Problem.  —  The  relationship  between  the  wave  length  (A)  the 
oscillation  frequency  (N)  and  the  velocity  of  electric  waves  (V)  has  already  been 
explained,  namely: 

V 


300,000,000 
Kence  for  the  600  meter  wave  N  =  -       -  =  500,000  cycles  per  second, 

600 

and  accordingly  for  300  meter  waves  N  =  1,000,000  cycles  per  second.  Then,  to  be 
placed  in  resonance  with  the  transmitter  aerial,  the  receiver  aerial  must  be.  ad  justed 
in  a  way  that  if  it  were  transmitting  electric  waves,  the  corresponding  oscillation 
frequencey,  in  the  case  of  the  600  meter  wave,  would  be  500,000  cycles,  and  for 
the  300  meter  wave,  1,000,000  cycles. 

Assume  that  the  receiver  antenna  has  been  adjusted  to  the  frequency  of  the 
transmitting  station  by  means  of  the  variable  elements  of  Fig.  147,  then  the  prob- 


Fig.    147a — Variable    Condenser   for    Receiver    Circuits. 

lem  remaining  is  to  make  audible  the  feeble  currents  induced  in  the  receiver  aerial 
or  at  least  to  make  known  their  presence  by  some  sort  of  a  current  indicator. 

The  ordinary  magneto  telephone  receiver  is  a  very  sensitive  indicator  of  electric  cur- 
rent, but  experiment  teaches  that  the  maximum  response  is  obtained  from  this  instrument 
when  an  alternating  current  or  an  equivalent  fluctuating  direct  current  of  a  frequency 
varying  between  300  and  500  cycles  per  second  flows  through  its  windings.  In  fact,  the 
movement  of  the  telephone  diaphragm  is  practically  nil  at  10,000  cycles  per  second,  and 
at  frequencies  of  the  order  of  one-half  million  per  second,  the  telephone  diaphragm  cannot 
keep  pace  with  the  reversals  of  flux.  And  even  if  it  could,  no  sound  would  be  heard, 
because  the  human  ear  will  not  respond  to  vibrations  in  excess  of  20,000  per  second. 

Another  factor  working  in  opposition  to  connecting  the  telephone  in  the  antenna 
circuit  directly,  is  the  fact  that  the  impedance  of  the  telephone  winding  will  not  permit 
high  frequency  currents  to  pass.  Hence  we  are  compelled  to  either  convert  the  current 
of  radio-frequency  into  a  uni-directional  pulsating  current  or  into  an  alternating  current 
of  a  frequency  within  the  range  of  the  human  ear. 

Currents  of  radio-frequency  induced  in  receiver  aerials  may  be  made  audible  by  means 
of  a  simple  rectifier — a  device  which  will  permit  the  preponderance  of  current  to  flow 
through  a  given  circuit  in  one  direction  only.  If  a  rectifier  is  placed  in  series  with  a  circuit 


132 


PRACTICAL   WIRELESS   TELEGRAPHY. 


A  A  A 


RECTIFIED 
CURRENT 


/if\  A  An 


TELEPHONE 
CURRENT 


Fig.    148a,    b, 


-Showing    how    Incoming:    Oscillations    are    converted    to 
Direct  Current  Pulses. 


in  which  a  current  of  radio-frequency  is  flowing,  the  latter  will  be  converted  into  a  direct 
or  pulsating  current.  Numerous  minerals  and  compounded  elements,  notably  among  these 
carborundum,  have  been  found  to  possess  the  property  of  rectification,  and  hence  a  group  of 
incoming  oscillations  such  as  shown  in  Fig.  148a  may  have  either  their  positive  or  negative 
currents  cut  off  as  in  Fig.  148b,  and  if  a  head  telephone  is  connected  in  series  with  the  rectifier, 
pulsating  direct  current  will  traverse  the  windings  as  shown  in  Fig.  148c. 

Now,  if  continuous 

•K  «  *        rAcr,, ,  *-r,  J?c"         oscillations  of  unvary- 

ing amplitude  were  in- 
duced in  the  receiver 
aerial,  the  rectified 
current  would  merely 
displace  the  telephone 
diaphragm  and  hold 
it  in  this  position  until 
the  current  is  turned 
off,  but  since  in  the 
damped  systems  of 
radiotelegraphy,  the 
oscillations  are  dis- 
continuous, or  occur 
in  groups,  the  current 
falls  to  zero  at  the  end 
of  each  group,  and, 
hence,  the  diaphragm 
of  the  telephone  is  re- 
leased, creating  a  sin- 
gle sound. 

126.  Simple  Receiver. — The  circuits  of  a  simple  radio  receiver  appear  in 
the  diagram  of  Fig.  149,  wherein  a  crystal  rectifier  D,  connected  in  series  with  the 
antenna  A,  is  shunted  by  the  receiving  telephone  P. 

The  action  of  this  apparatus  during  the  reception  of  signals  may 
be  explained  as  follows :  A  train  of  waves  radiated  by  the  trans- 
mitter induces  an  alternating  current  in  the  aerial  circuit  which 
will  flow  freely  through  the  crystal  in  one  direction,  but  will  be 
opposed  in  the  opposite  direction.  In  one  direction,  let  us  say,  the 
current  passes  from  the  earth  upward  through  the  crystal,  and  thus 
places  a  charge  on  the  aerial  wires,  but  the  return  current  is  op- 
posed; hence  the  rectified  oscillations  (for  each  spark  of  the  trans- 
mitter) accumulate  a  charge  on  the  antenna  wires,  which  at  the 
termination  of  a  wave  train  leaks  to  earth  through  the  head  tele- 
phone, creating  a  single  sound  for  each  group  of  incoming 
oscillations. 

Because  of  its  resistance,  the  crystal  impedes  the  free  flow  of 
oscillations  and,  therefore,  to  some  extent,  destroys  the  tuning 
qualities  of  the  aerial  circuit.  Hence,  to  enhance  the  tuning  prop- 
erties of  the  antenna  system,  the  crystal  is  removed  therefrom  and 
connected  in  a  second  circuit,  termed  the  "local  detector"  circuit. 

127.  The  Inductively  Coupled  Receiver. — The  two 
circuit  receiver  is  shown  in  the  diagram  of  Fig.  150,  where- 
in a  coil  L-l,  connected  in  the  antenna  circuit,  performs  the 
double  function  of  adjusting  the  frequency  of  the  antenna 
system  to  resonance  with  the  transmitter  and  transferring 
the  induced  oscillations  to  a  secondary  circuit,  consisting  of 
the  coil  L-2  shunted^  by  the  variable  condenser  C-2.  More 
clearly,  the  second  circuit  is  coupled  inductively  to  the  first 
circuit.  A  crystal  rectifier,  D,  and  head  telephone,  P,  joined 


Li 


Fig.      149— Circuit     of 
Simple    Radio     Receiver. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


133 


Fig.     150 — Fundamental    Diagram    of    an    Inductively 
Coupled    Receiver. 


in  series,  are  connected  across  the  terminals  of  the  secondary  condenser.  The  two 
circuits  are  distinguished  in  the  following  manner :  The  circuit  comprising  the  aerial 
and  the  primary  winding  of  the  receiving  tuner  L-\  is  called  the  open  oscillation 

circuit,  and  the  circuit  comprising  the 

^  7  secondary   coil  L-2   and  the   variable 

Nlr      ,  .          ,  .  condenser    C-2,    is    called   the   closed 

oscillation  circuit.  They  are  also 
termed  the  primary  and  secondary  cir- 
cuits, respectively. 

The  current  induced  in  the  receiving 
aerial  is  made  audible  in  the  receiving 
telephone  in  the  following  manner :  When 
oscillations  flow  in  the  receiving  aerial, 
an  alternating  current  passes  through  L-l 
and  a  corresponding  alternating  magnetic 
field  cuts  through  the  turns  of  L-2,  induc- 
ing therein  a  current  of  radio-frequency 
which  is  built  up  in  amplitude  by  placing 
the  two  circuits  in  electrical  resonance. 
The  variable  condenser  C-2  not  only  permits  the  closed  circuit  to  be  adjusted  to  resonance 
with  the  open  circuit,  but  also  permits  a  closeness  of  adjustment  which  the  usual  secon- 
dary inductance  changing  switch  does  not  afford.  If  the  coupling  between  the  primary 
and  secondary  windings  is  relatively 
"loose,"  rather  large  amounts  of  capacity 
and  small  amounts  of  inductance  are  em- 
ployed in  the  closed  circuit,  for  the  best 
strength  of  signal,  but  if  the  coupling  be- 
tween the  windings  is  close,  small  values 
of  capacity  and  rather  large  values  of  in- 
ductance are  the  factors  which  contribute 
to  the  maximum  strength  of  signal. 

The  oscillations  built  up  in  the  sec- 
ondary circuit  by  adjusting  it  to  reso- 
nance with  the  antenna  circuit,  overflow 
to  the  shunt  detector  circuit,  where  part 
of  the  current  is  rectified  by  the  crystal 
detector  D  and  stored  up  in  the  con- 

r>  «        OM.         i.  i    ±   j     •  Fig-    151— The   Conductively   Coupled  Receiver. 

denser    C-l.      Ihe    charge    accumulated   in 

the  latter  during  the  time  of  a  single  train  of  oscillations,  discharges  through  the  head 
telephone  P,  causing  the  telephone  diaphragm  to  vibrate  at  a  rate  corresponding  to  the 
spark  frequency  of  the  transmitter. 

128.  Other  Methods  of  Coupling. — Tf  the  open  and  closed  oscillation  cir- 
i    -i    ,  J£P\.  cu*ts  °f a  receiver  are  coupled  through 

\  /  xQ  (j.  an  auto-transformer  as  in  Fig.  151,  the 

two  circuits  are  then  said  to  be  di- 
rectly or  conductively  coupled. 

In  this  system  part  of  the  turns  of  a 
single  coil  constitute  the  primary  cir- 
cuit and  part  the  secondary  circuit;  the 
turns  bet-ween  the  points  A  and  B  may  be 
considered  as  the  primary  winding  of  the 
tuner,  while  those  from  B  to  C  may  be 
said  to  constitute  the  secondary  winding. 
Now  the  oscillation  frequency  of  the  an- 
tenna circuit  can  be  increased  or  decreased 
by  the  variable  contact  A  and  similarly 

Fig.    152— The   Capacitively   Coupled  Receiver.  f        th     d   t     t          '        't  b  t      t  C      Fun 

damentally  the  operation  of  this  apparatus  is  similar  to  the  inductively  coupled  receiver 
with  the  exception  that  part  of  the  current  induced  in  the  antenna  circuit  flows  directly 
through  the  shunt  path  afforded  by  the  detector  circuit.  The  disadvantage  of  this  method 


W 


* 


134 


PRACTICAL   WIRELESS   TELEGRAPHY. 


of  coupling  is  that  any  change  of  wave-length  adjustment  of  the  open  and  closed  circuits 
changes  the  coupling,  but  this  can  be  prevented  by  connecting  an  aerial  tuning  inductance 
in  series  with  the  primary  winding.  The  primary  and  secondary  windings  may  be  "loosely" 
coupled  at  any  particular  wave  length  adjustment,  by  adding  turns  at  the  aerial  tuning 
inductance  and  taking  them  out  at  the  primary  winding.  This  will  decrease  the  mutual 
inductance  of  the  two  circuits  and  therefore  will  reduce  the  coupling. 

The  method  termed  the  capacitive  coupling  is  shown  in  the  diagram  of  Fig.  152.     The 
primary  and   secondary  coils   shown  at   L-l   and   L-2  are  not   in   direct  inductive   relation. 


Pi  B, 


Fig.   153a,  b,  c — Complete  Circuit  for  a  Carborundum  Rectifier  and  Receiving  Tuner. 


They  are  said  to  be  electrostatically  coupled  through  the  condensers  C-l  and  C-2.  In 
practice,  the  condensers  C-l  and  C-2  are  mounted  on  a  single  shaft  and  their  capacity 
varied  simultaneously  by  a  single  control  knob.  It  is  claimed  that  since  a  fixed  potential 
exists  across  coil  L-l  the  energy  transferred  to  the  secondary  circuit  varies  as  the  capacity 
of  condensers  C-l  and  C-2.  Now  the  greater  the  coupling  the  greater  will  be  the  transfer 
of  energy  from  the  antenna  to  the  detector  circuit  and  hence  the  coupling*  between  the 


*The  author  has  had  practically  no  experience  with  the  capacitive  receiver  and  therefore  cannot  speak 
authoritatively    on  -the    working    of    this    circuit. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


135 


circuits  varies  as  the  capacity  of  the  coupling  condensers.  It  is  claimed  this  circuit 
possesses  the  advantage  that  a  fixed  degree  of  coupling  between  the  primary  and 
secondary  windings  can  be  held  while  the  wave  length  adjustment  of  the  antenna  and  local 
detector  circuits  is  being  varied  throughout  the  range  of  the  tuner.  It  is  probable,  however, 
that  this  statement  requires  qualification. 

In  general  the  student  will  note  from  the  foregoing  that  in  any  complete 
wireless  telegraph  system  four  circuits  must  be  adjusted  to  or  tuned  to  the  same 
frequency  of  oscillation  in  order  that  communication  may  be  established,  as 
follows:  At  the  transmitting  station  the  closed  and  open  oscillation  circuits  are 
adjusted  for  resonance.  At* the  receiving  station  the  receiving  aerial  circuit  is 
adjusted  to  the  frequency  of  the  oscillations  of  the  transmitter  and  the  local  or 
secondary  circuit  adjusted  to  resonance  ivith  the  receiving  aerial  circuit;  and 
for  maximum  response,  the  receiving  detector  must  be  adjusted  to  its  maximum 
state  of  sensitiveness. 

129.  The  Carborundum  Detector  and  Tuning  Circuits. — The  most  widely 
used  of  all  detectors  is  the  carborundum  crystal  rectifier,  the  tuning  circuits  for 
which  are  shown  in  Fig.  153a,  b,  c.    These  diagrams  indicate  as  well  the  apparatus 
included  in  a  modern  receiving  set.     In  addition  two  modified  circuits  showing 
the  connection  of  the  potentiometer  in  various  modern  tuners,   are  presented. 
Before  proceeding  with  an  explanation  of  the  circuits  of  Fig.  153a,  the  function 
of  the  potentiometer  will  be  explained. 

The  application  of  a  weak  battery  current  to  the  carborundum  crystal  and  head  tele- 
phone circuit  has  been  found  to  have  a  marked  effect  on  the  intensity  of  the  incoming 
signals,  but  the  strength  of  the  current 
must  be  carefully  and  closely  regulated 
and  must  be  passed  through  the  crystal  in 
a  definite  direction  to  secure  the  maximum 
response.  Ignoring  for  the  moment  an 
explanation  of  the  function  of  the  local 
current  in  a  radio  receiver,  the  reader 
should  note  the  diagram,  Fig.  154,  showing 
the  connections  of  the  potentiometer  to  a 
local  battery. 

A  resistance  coil  A,  D,  is  connected  to 
the  terminals  of  a  2  or  4  volt  battery  B-l. 
A  shunt  resistance  R  has  the  variable  con- 
nection B  which  is  generally  a  sliding  con- 
tact. R  may  be  taken  to  represent  a  car- 
borundum rectifier.  According  to  the  law 
of  divided  circuits,  the  maximum  E.  M.  F. 
is  maintained  across  R  when  B  is  shifted 
to  the  end  D,  but  in  the  direction  opposite  or  towards  A,  the  E.  M.  F.  gradually  reduces 
to  zero. 

In  the  circuit  originally  adopted  for  the  carborundum  crystal,  the  poten- 
tiometer and  detector  were  connected  as  in  Fig.  153-a,  but  the  modified  circuits 
of  Fig.  153-b  and  Fig.  153-c  are  also  in  use.  In  Fig.  153-b  the  telephone  and 
potentiometer  are  shunted  across  the  stopping  condenser  C-2  but  in  Fig.  153-c 
the  potentiometer  is  connected  in  series  with  the  crystal  rectifier.  Since  the 
resistance  of  the  crystal  exceeds  that  of  the  potentiometer  by  several  thousand 
ohms,  the  resistance  of  the  latter  has  little  effect  on  the  strength  of  the  incoming 
signal.  Although  practically  equal  results  are  obtained  with  either  connection, 
the  circuit  of  Fig.  153-b  is  pointed  out  by  some  investigators  to  be  the  one  which 
gives  the  maximum  response. 

130.  Adjustment  of  the  Inductively  Coupled  Tuner. — During  the  reception 
of  wireless  telegraph  signals  the  inductive  receiving  tuner  may  be  adjusted  to 
resonance  with  the  sending  station  in  the  following  manner : 

If  the  secondary  circuit  is  calibrated  in  wave  lengths  corresponding  to  various  positions 
of  the  secondary  condenser  and  inductance  switch,  and  a  table  of  wave  lengths  supplied, 


2  VOLTS 


wwwvwwww 


4  VOLTS 


II 


Fig.    154 — Connections    of    Potentiometer    for    Crystal 
Detector. 


136  PRACTICAL   WIRELESS   TELEGRAPHY. 

the  secondary  circuit  may  be  set  directly  to  the  wave  length  of  the  incoming  signal,  taking 
care  to  select  large  values  of  inductance  and  small  values  of  secondary  capacity  for  any 
particular  wave.  The  secondary  winding  should  then  be  placed  in  partial  inductive  relation 
to  the  primary  winding,  followed  by  varying  the  capacity  and  inductance  of  the  antenna 
circuit  until  response  is  obtained.  This  is  to  be  followed  by  adjusting  the  sliding  contact 
on  the  potentiometer  and  trying  new  points  on  the  crystal  rectifier  until  the  loudest  signals 
are  obtained  in  the  head  telephone. 

Because  the  capacity  and  inductance  of  ships'  aerials  differ  considerably  it  is  evident 
that  a  given  primary  winding  will  afford  different  wave  length  adjustments  with  different 
aerials,  consequently,  the  primary  inductance  changing  switch  cannot  be  calibrated  directly 
in  wave  lengths  previous  to  installation.  But  after  the  receiver  is  installed,  the  primary 
and  secondary  circuits  both  can  be  readily  calibrated  by  comparison  with  a  wavemeter 
and  the  adjustment  for  any  particular  wave  length  quickly  duplicated  whenever  required. 

In  event  that  a  receiving  tuner  is  not  calibrated,  the  receiver  can  be  tuned  to  a  given 
transmitter  in  the  following  manner: 

(1)  Set  the  secondary  circuit  approximately  to  the  required  wave  length  using 
large  values  of  inductance  and  small  values  of  capacity; 

(2)  Place  the  secondary  inductance  in  close  inductive  relation  to  the  primary 
inductance ; 

(3)  Add  inductance  in  the  antenna  circuit  until  response  is  obtained  in  the 
head  telephones; 

(4)  Adjust  detector  to  maximum  degree  of  sensibility  by  potentiometer; 

(5)  Then  reduce  coupling  between  primary  and  secondary  windings; 

(6)  Try  new  values  of  inductance  and  secondary  condenser,  also  new  values  in 
the  aerial  circuit; 

(7)  Adjust  in  this  manner  for  maximum  strength  of  signals  or  until  interfer- 
ence is  eliminated. 

(a)  Theory  of  adjustment.  Part  of  the  energy  of  the  oscillations  induced  in  the  receiver 
aerial  is  lost  by  the  resistance  of  the  antenna  conductors,  part  by  re-radiation  of  the  energy 
in  the  form  of  a  wave  motion  and  the  remainder  through  transference  to  the  local  detector 
circuit.  The  energy  imparted  to  the  detector  circuit  is  useful  energy  because  it  produces  the 
response  in  the  head  telephone,  but  the  energy  extracted  in.  this  manner  also  has  a  marked 
effect  on  the  tuning  qualities  of  the  receiving  antenna,  which  must  be  taken  into  account. 

This  may  be  explained  as  follows :  If  the  secondary  circuit  is  coupled  loosely  to  the 
primary  circuit,  small  amounts  of  energy  will  be  extracted  from  the  antenna  .oscillations ; 
hence,  the  antenna  will  oscillate  with  greater  persistence  and  will  only  respond  with  free- 
dom to  electric  waves,  the  frequency  of  which  coincides  with  its  natural  frequency  of 
oscillation.  On  the  other  hand,  if  the  primary  and  secondary  circuits  are  coupled  closely, 
a  greater  amount  of  energy  will  be  extracted  from  the  incoming  oscillations  and  they  will 
therefore  be  damped  out  more  rapidly  than  under  conditions  of  close  coupling.  The  re- 
ceiving aerial  will,  under  these  conditions,  respond  to  waves,  the  frequency  of  which  may 
be  somewhat  greater  or  less  than  the  natural  frequency  of  the  aerial  circuit. 

Thus  it  is  seen  that  if  the  two  circuits  are  closely  coupled,  the  receiver  circuits  will  tune 
"broadly"  or  if  coupled  loosely,  the  circuits  will  tune  "sharply,"  that  is  to  say,  when  the 
receiving  transformer  coils  are  loosely  coupled,  a  small  change  of  inductance  or  capacity 
will  eliminate  the  signals  of  a  given  station,  but  when  the  receiver  is  "tightly''  coupled,  a 
much  larger  change  of  inductance  or  capacity  will  be  required  to  eliminate  the  signals. 
This  can  be  stated  in  another  way  by  saying  that  the  change  of  coupling  between  the 
primary  and  secondary  circuits  alters  the  effective  resistance  of  the  antenna  and,  therefore, 
has  direct  influence  on  the  damping  of  the  antenna  oscillations. 

It  has  been  shown  by  several  investigators  that  maximum  response  is  obtained  in  a 
receiving  apparatus  so  adjusted  that  the  resistance  of  the  primary  and  secondary  circuits 
are  equal  and  since  the  effective  resistance  of  any  receiving  system  varies  widely  according 
to  the  design  of  the  aerial,  the  earth  plate  resistance  and  the  type  of  receiving  apparatus, 
it  is  clear  that  some  particular  degree  of  coupling  of  the  tuner  will  give  the  maximum 
response  in  the  head  telephone. 

The  two  circuit  receiver  with  inductive  coupling  permits  the  receiving  operator  to 
eliminate  the  signals  from -interfering  stations;  in  fact  by  judicious  adjustment  (of  the 
coupling)  signals  of  the  same  wave  length  can  be  eliminated  provided  they  have  different 
decrements.  Generally  an  advancing  wave  of  feeble  decrement  permits  the  primary  and 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


137 


secondary  circuits  to  be  loosely  coupled  and  conversely  a  highly  damped  wave  requires  the 
receiver  circuits  to  be  closely  coupled. 

The  antenna  circuit  at  any  receiving  station  can  be  made  to  tune  more  sharply  by 
connecting  a  variable  condenser  in  series  and  adding  inductance  until  the  wave  length 
adjustment  for  resonance  is  obtained.  This  decreases  the  natural  decrement  of  the  re- 
ceiving aerial  but  generally  results  in  decrease  of  the  strength  of  signals  but  the  loss  of 
signal  strength  is  more  than  compensated  for  by  the  degree  of  selectivity  obtained. 

The  sharpest  tuning  or  the  highest  degree  of  selectivity  is  obtained  in  the  local  detector 
circuit  when  the  variable  condenser  in  shunt  to  the  secondary  winding  is  worked  at  small 
values  of  capacity  with  correspondingly  large  values  of  inductance  for  a  given  wave 
length ;  but  if  loose  coupling  is  employed  greater  values  of  capacity  and  lesser  values  of 
inductance  may  increase  the  selectivity. 

As    will    be    seen    from    the    foregoing 

0  explanations,  interference  can  be  prevented 

\^          at  the  receiving  station:     (1)    by  employ- 
^^^         ing    loose    coupling    between    the    primary 

I  "  ^^^»         and  secondary  windings;   (2)   by  inserting 

a  condenser  in  the  antenna  circuit  and 
adding  inductance  until  resonance  is  se- 
cured for  a  given  wave  length. 

Irrespective  of  the  selectivity  obtained 
by  radio-frequency  tuning,  spark  trans- 
mitters having  different  spark  notes  tend 
to  prevent  interference.  By  skill  obtained 
through  practice,  the  receiving  operator 
can  pick  out  a  spark  note  of  particular  pitch  and  translate  the  signals  through  the  inter- 
ference of  one  or  more  stations,  the  more  so  if  the  spark  note  of  the  station  desired  has  a 
distinctive  pitch.  Occasionally  interfering  stations  can  be  tuned  out  by  detuning  the  closed 
and  open  circuits,  but  signals  will  only  be  received  in  this  way  when  the  receiving  station 
is  situated  near  to  the  transmitting  station. 

131.  The  Action  of  the  Carborundum  Crystal. —  Before  proceeding  with  an 
explanation  of  various  detectors  and 
the  circuit  best  adapted  to  their 
operation,  we  will  briefly  consider  a 
generally  accepted  explanation  of  the 
operation  of  the  carborundum  de- 
tector and  the  function  of  the  local 
battery  current  during  the  reception 
of  radio  signals. 

The  student  is  now  aware  that  if  we 
have  a  circuit  such  as  shown  in  Fig. 
154a,  consisting  of  a  variable  source  of 


Fig.      154a — Circuit      for      Determining      Volt-ampere 
Characteristic    of  a   Given    Conductor. 


6 


VOLTS 


155 — Characteristic     Curve     of     Ordinary 
sistance. 


direct  current  B  to  which  is  connected  a 
resistance  R  and  an  ammeter  A  in  series 
and  if  the  voltage  at  B  is  progressively 
increased,  the  flow  of  current  as  indicated  by  the  ammeter,  increases  in  the  direct  ratio  in 
accordance  with  Ohm's  law.  Furthermore,  if  we  plot  the  results  of  this  experiment  in  the 
form  of  a  graph  on  cross  section  paper,  as  in  Fig.  155,  we  find  that  a  line  drawn  common  to 

all    co-ordinate    points    located    in    accord- 
ance with   the   data,   will   be   straight   and 

f —  }£J  1          uniform.     Let  it  be  noted  from  this,  curve 

^*k  that  if  the  voltage  be  doubled,  the  current 

~"~^~~  is  doubled  and  so  on. 

Now  if  we  substitute  for  the  resistance  R 
a  crystal  of  carborundum  D  as  in  Fig.  156, 

Twe  find  first,  that  the  current  in  one  direc- 
tion   with    a    given    impressed    voltage    is 
)          much   greater   than   when   passed    through 

the   crystal   in   the   opposite  direction    and 
second,    that   if   the   current    flows    in    the 


Fig.     156 — Apparatus     for     Determining    Volt-ampere 
Characteristic     of     Carborundum     Rectifier* 


*Diagrams    Figs.    155    and    156    should   have    a   voltmeter    in    shunt   to    the    source    of    E.    M.    F.    and    a 
regulating  resistance   in   series. 


138 


PRACTICAL  WIRELESS  TELEGRAPHY. 


direction  of  best  conductivty  and  the  voltage  again  progressively  increased,  the  results  do 
not  accord  with  Ohm's  law  as  the  curve,  Fig.  157,  clearly  indicates,  e.  g.,  the  current  is  not 
proportional  to  the  voltage.  It  will  be  seen  that  at  first  the  current  does  not  increase  as 
rapidly  as  it  should  to  be  in  accord  with  Ohm's  law  and  after  a  certain  critical  value  of 
voltage  is  passed  the  current  exceeds  the  value  to  be  expected  by  the  same  law.  A  curve 
of  this  type  is  said  to  have  a  rising  characteristic. 

We  may  now  assume  this  curve  to  apply  to  the  detector  circuit  of  Fig.  153a  and  that 
signals  are  being  received  from  a  given  station.    Let  the  E.  M.  F.  of  the  battery  be  three 

volts.     From  the  curve, 

20 Fig.    157,   we   note   that 

the  corresponding  cur- 
rent is  6  microamperes. 
Now  let  the  alternating 
current  of  radio-fre- 
quency (the  incoming 
signal)  have  for  pur- 
poses of  illustration  a 
potential  of  one  volt  and 
let  it  be  superposed  upon 
the  battery  current  flow- 
ing through  the  crystal. 
Then  in  one  direction  1 
volt  will  be  added  to  3 
volts  and  from  the 
curve,  the  current  corre- 
sponding to  4  volts  is 
16  microamperes.  But 
when  the  alternating 

E.  M.   F.   flows   in    the 
opposite  direction  it  op- 
poses   the    local    battery 
and  the  resultant  E.  M. 

F.  is  2  volts.     Reading 
from  the  curve  we  obtain 
a    current    value    of   2.5 
microamperes. 

It  will  thus  be  seen 
under  the  influence  of 
the  impressed  alternat- 
ing E.  M.  F.  that  the  cur- 


1234 
VOLTS 

Fig.    157 — Characteristic    Curve    of   Carborundum    Rectifier. 


rent  in  the  local  battery  circuit  varies  between  2.5  and  16  microamperes,  but  the  sound 
produced  by  the  head  telephone,  as  will  be  explained,  is  proportional  to  the  difference  between 
the  normal  current  flowing  through  the  crystal  and  the  average  value  of  current  flowing 
when  an  external  oscillating  voltage  is  applied.  This,  perhaps,  can  be  more  clearly 
explained  by  means  of  the  curves  shown  in  Fig.  157a,  where  the  effect  of  superposing  the 
antenna  oscillations  on  the  local  battery  current  over  the  duration  of  a  single  train  of  incom- 
ing oscillations  is  shown. 

As  shown  in  Fig.  157,  the  normal  current  flowing  through  the  head  telephone,  when  no 
oscillating  voltage  is  applied,  is  6  microamperes  (the  voltage  of  the  battery  being  3  volts), 
but  when  an  oscillating  current  of  1  volt  is  added  on  that  of  the  local  battery,  the  maximum 
amplitude  of  the  initial  oscillation  (Fig.  157a)  in  the  wave  train  is  16  microamperes,  and, 
of  course,  successive  maxima  will  be  of  lesser  amplitude  according  to  the  decay  of  the 
wave  train  (as  shown  by  the  series  of  decaying  maxima).  We  see  also  that  the  suc- 
cessive reductions  of  the  normal  battery  current  (shown  below  the  line  A,  B)  are  rela- 
tively small  because,  as  shown  in  Fig.  157,  for  all  voltages  less  than  3  volts  the  flow  of 
current  (in  microamperes)  through  the  local  circuit  is  relatively  weak.  The  result  of  this 
it  seen  to  be  a  series  of  positive  maxima  of  gradually  decreasing  amplitude  to  which 
the  telephone  diaphragm  cannot  respond  individually  but  which  produce  an  average  effect 
in  the  receiver.  The  average  current  in  the  case  of  Fig.  157a  may  be  considered  for  mere 
illustration  to  be  9  microamperes  and  the  difference  between  the  normal  current  6  micro- 
amperes and  the  average  current  9  microamperes  (or  3.5  microamperes)  is  the  current 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


139 


which  produces  audible  sound  in  the  receiving  telephone.     We  may  state  the  foregoing  in 
another  way  by  saying  that  the  added  voltage  due  to  the  oscillating  E.  M.  F.  being  im- 


C-D    APPROXIMATE    AVERAGE    TELEPHONE.    CURRENT 
A- 6   NORMAL    6ATTERY     CURRENT 


BALL  JOINT 


Fig.     157a. — Curves     Showing     the     Fluctuations     of     the     Local     Battery     Current     flowing 
through     the     Carborundum    Rectifier    during    the    Reception    of    Signals. 

pressed  upon  the  crystal  is  greater  than  the  subtracted  voltage  and  that  the  final  effect  of 
this  is  an  increase  of  current  through  the  head  telephone  circuit  over  the  duration  of  one 
wave  train. 

From  the  foregoing  explanation 
it  is  apparent  that  when  an  operator 
at  a  given  receiving  station  adjusts 
the  position  of  the  sliding  contact 
on  the  potentiometer  for  maximum 
signals,  he  adjusts  the  flow  of  cur- 
rent through  the  crystal  and  head 
telephone  to  correspond  to  the 
critical  point  on  the  characteristic 
curve,  or,  in  other  words,  to  that 
point  on  the  curve  where  the  super- 
position of  a  slight  antenna  E.  M. 
F.  upon  the  local  E.  M.  F.  causes  a 
relatively  large  change  in  the 
strength  of  the  local  battery  cur- 
rent. And  it  also  follows  that  the 
steeper  the  characteristic  curve  of 
a  given  crystal,  the  greater  will  be 
the  change  of  current  for  a  given 
impressed  alternating  voltage.  In 
other  words,  the  crystal  with  a 
steep  curve  will  give  the  loudest 
signals  and  is  said  to  be  "more  sen- 
sitive" to  incoming  oscillations. 

132.  Adjustment  of  Crystal 
Detectors. — No  specific  rule  for  lo- 
cating the  sensitive  spot  on  a  crystal 
rectifier  can  be  laid  down;  in  fact,  the 
pressure  and  position  of  the  opposing 
contact  for  maximum  signals  can  only 
be  determined  by  experiment.  Car-  Fig.  158— Carborundum  Detector  Complete. 


CARBORUNDUM 

/CRYSTAL 


LSTEEL  CONTACT  POINT 


140 


PRACTICAL   WIRELESS   TELEGRAPHY. 


BORNITE     /ZINCITE 


Fig.    159 — The    Zincite    Bornite    Detector. 


borundum  crystals  as  a  whole  require  greater  pressure  at  the  opposing  contact  than  crystals 
of  galena,  silicon,  etc.,  although  certain  crystals  under  observation  have  required  exceed- 
ingly light  contact  pressure.  With  all  crystals  employing  a  local  battery,  it  is  important 
that  the  local  current  flow  in  a  certain  direction  and  that  its  strength  be  carefully  regulated. 
This  means  in  practice  that  either  the  connections  to  the  battery  must  be  reversed,  or  the 
crystal  must  be  turned  about  in  the  holder  and  left  at  the  position  in  which  the  loudest 
signals  are  obtained  from  a  given  sending  station.  The  potentiometer,  of  course,  must  be 
adjusted  simultaneously. 

The  circuit  of  Fig.  153-b  is  suitable  for  the  zincitc-bornite  detector  provided  a  fixed 
resistance  of  about  2,000  ohms  is  connected  in  series  with  the  battery  B-l  and  the  poten- 
tiometer P-l.  Certain  crystals  of  this 
combination  respond  better  with  a  local 
battery  while  others  do  not  require  it, 
but  with  practically  any  crystal  it  aids  at 
least  in  obtaining  the  sensitive  adjust- 
ment to  employ  a  local  battery,  e.  g.,  the 
sensitive  spot  can  be  more  quickly 
located. 

If  the  test  buzzer  to  be  described 
further  on  is  employed,  a  crystal  can  be 
adjusted  to  sensitiveness  whether  or  not 
the  distant  transmitter  is  in  opera- 
tion. 

133.  Detector  Holders. — Two  types  of  crystal  holders  appear  in  figs.  158 
and  159.    The  first  is  suitable  for  the  carborundum  crystal  which  is  mounted  in  a 
small  brass  cup  with  some  form  of  soft  metal  such  as  Wood's  metal.     A  small 
point,   such   as   a   steel   phonograph   needle,   is   mounted   on   the   movable   arm 
and  makes  contact  with  the  crystal.   During  adjustment  this  point  is  "jabbed  in" 
at  various  points  on  the  crystal  until  a  sensitive  spot  is  located. 

A  detector  holder  suitable'  for  the  zincite-bornite  detector  is  shown  in  Fig.  159.  The 
large  cup  is  filled  with  5  or  6  zincite  crystals  while  the  opposite  cup  carries  a  crystal 
of  bornite.  The  cup  is  fastened  to  the  end  of  the  arm  with  the  universal  joint.  The 
crystal  of  bornite  may  thus  be  placed  in 
contact  with  the  surface  of  any  of  the 
zincite  crystals  being  shifted  from  one  to 
the  other  until  the  adjustment  for  maxi- 
mum strength  of  signals  is  found. 

A  variety  of  crystals  holders  have 
been  designed  for  crystals  of  galena,  sili- 
con, cerusite,  etc.  One  type  appears  in 
Fig.  160,  where  the  crystal  is  held  in  a 
small  cup  by  three  screws.  A  light  wire 
contact  mounted  on  the  movable  arm 
bears  with  slight  pressure  on  the  surface 
of  the  crystal. 

134.  Classification  of  the  Receiving  Detectors. — The  receiving  detectors 
of  wireless  telegraphy  differ  greatly  both  in  point  of  mechanical  construction  ana 
mode  of  operation,  and,  in  addition,  they  possess  widely  varying  degrees  of  sen- 
sitiveness.    Certain  types,  for  instance,  are  highly  sensitive  to  electrical  oscilla- 
tions but  are  difficult  to  keep  in  permanent  adjustment;  others  are  less  sensitive 
but  possess  marked  degrees  of  stability.    Still  others  are  in  the  nature  of  a  com- 
promise  and  may   occupy  approximately   a   position   midway  between   the  two 
extremes. 

Some  receiving  detectors  rely  upon  the  principal  of  rectification  (as  we  have 
already  shown)  and  will  convert  an  alternating  current  of  radio-frequency  to  a 
uni-directional  current;  others  have  the  property  of  rectification  combined  with 
the  ability  to  vary  a  local  source  of  battery  current  in  a  manner  much  similar  to 
the  working  of  an  ordinary  telegraph  relay.  The  operation  of  certain  other  detec- 
tors is  based  upon  the  influence  of  electrical  oscillations  upon  magnetised  iron  or 


Fig.     160 — Detector    Holder    for    Galena    and    Silicon 
Crystals. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS.  141 

upon  the  ability  of  these  Oscillations  to  cause  certain  granulated  metals  to  cohere. 
Perhaps  the  most  highly  developed  of  all  receiving  detectors  are  the  vacuum 
tube  rectifiers  which  are  extremely  sensitive  and  possess  the  important  property 
of  amplifying  the  signals  when  connected  in  cascade.  Certain  types  of  vacuum 
amplifiers  will  remain  in  a  sensitive  state  of  adjustment  over  an  indefinite  period 
or  throughout  their  length  of  life. 

It  should  be  kept  in  mind  that  the  most  sensitive  receiving  detector  is  not  always  the 
one  most  convenient  or  the  most  practical  for  commercial  use.  For  instance,  extremely 
-sensitive  amplifying  detectors  may  require  intricate  apparatus,  having  so  many  points  of 
adjustment  that  its  manipulation  may  call  for  the  services  of  a  highly  skilled  engineer  and 
further  the  circuits  may  be  of  such  a  type  that  the  apparatus  cannot  be  quickly  changed 
from  one  wave  length  to  another.  In  event  of  the  latter,  the  tuner  would  be  of  little  value 
for  marine  service  which  requires  the  receiver  to  be  one  capable  of  quick  adjustment 
to  land  stations  operating  at  various  wave  lengths.  Furthermore,  extremely  sensitive  de- 
tectors bring  in  a  certain  amount  of  interference  from  far  distant  stations  which  would  not 
be  heard  on  less  sensitive  detectors. 

The  most  practical  detector  for  commercial  working  is  one  that  combines  a  fair  degree 
of  sensibility  with  ruggedness  and  stability"  of  adjustment,  and  so  far  among  the  crystal 
detectors  none  has,  in  this  respect,  excelled  the  carborundum  rectifier.  The  Marconi  mag- 
netic detector  is  universally  recognized  as  being  the  most  stable  and  "foolproof"  of  all  re- 
ceivers but  it  lacks  sensitiveness  on  the  shorter  waves.  The  vacuum  valve  detectors,  on 
the  other  hand,  are  considered  to  be  the  most  sensitive  among  commercial  receivers,  but 
they  possess  the  disadvantage  of  requiring  complicated  circuits  for  best  results. 

To  impress  upon  the  reader's  mind  the  utility  of  the  various  types  of  receiving 
detectors,  we  may  classify  them  under  five  general  headings.  Under  the  first 
heading  we  may  name  the  detectors  which  require  no  local  battery  and  under  the 
second  heading,  those  detectors  in  which  the  response  in  the  telephone  depends 
upon  the  application  of  a  local  battery  current  as  well  as  upon  the  current  of  the 
incoming  oscillations.*  Certain  detectors  may  be  classified  under  both  headings 
because  they  may  function  to  some  extent  with  or  without  a  local  battery. 

In  addition  we  may  note  under  a  third  heading  the  detectors  considered  as 
rectifiers  of  radio-frequent  currents  and  in  a  fourth  and  fifth  headings,  those 
suitable  for  response  to  either  damped  or  undamped  oscillations  respectively.  It 
is  to  be  noted  that  a  few  types  come  under  all  headings. 

Detectors    Functioning  f  Galena-Silicon-Zmcite    Bornite-Carborundum     (sat- 
Without  Local  Battery  I   lsfactory  for  short  distance  receiving)— Fleming  Valve 

(_  (filament  battery  only). 
Detectors     which     depend  C 

upon  the  combined  effects  of  I  Carborundum— Zincite  Bornite  (sometimes  used  with 
received  energy  and  local  ]  local  battery)— Fleming  Valve  (with  local  battery)— 
battery  current.  [  Three  Element  Valve— Silicon. 

f  Galena  —  Silicon  —  Carborundum  —  Cerusite — Zincite- 
%££?**      Va,ve-Three      Element     Valve- 

{Galena  —  Silicon  —  Zincite  Bornite  —  Carborundum 
— Fleming  Valve — Three  Element  Valve — Marconi 
Magnetic-(Tikker,  Tone  Wheel  and  Heterodyne  sys- 
tem will  give  some  response  from  spark  transmitters, 
but  are  not  satisfactory  for  such  reception). 

Detectors     of     Undamped  f  Tikker — Tone    Wheel — Heterodyne    Receiver — Vacuum 
Oscillation.  \  Valve  Oscillator. 

135.  Fleming  Valve  Detector  and  Tuning  Circuits. — A  receiving  detector 
of  notable  merit  is  the  Fleming  oscillation  valve  the  action  of  which  is  primarily 

*Footnote:  We  should  be  careful  to  distinguish  between  detectors  requiring  a  "local  battery"  and 
those  requiring  a  local  "source  of  energy."  For  example,  the  Tikker,  the  Tone  Wheel  and  the  Marconi 
Magnetic  detectors  require  a  local  driving  force,  which  in  reality  take  the  place  of  the  local  battery;  also 
the  Heterodyne  receiver  requires  a  local  source  of  radio-frequent  currents  upon  which  the  response  in  the 
receiver  depends  as  well  as  the  current  induced  in  the  receiver  aerial. 


142 


PRACTICAL   WIRELESS   TELEGRAPHY. 


based  on  the  emission  of  electrons  from  highly  heated  metals  in  vacua.  In  its 
commercial  form  it  consists  of  a  highly  exhausted  glass  bulb,  Fig.  161,  containing 
a  4  or  12  volt  lamp  filament,  F,  of  carbon,  platinum  or  tungsten  surrounded  by  a 
metal  plate  or  cylinder  P-2  of  copper,  nickel,  etc.,  from  which  a  connection  is 
extended  to  the  outside  of  the  bulb. 

When   the   filament   is   brought  to  a   state   of   incandescence   by  a  battery    B,  negative 
electricity  can  pass  from  the  filament  to  the  plate,  but  not  in  the  opposite  direction,  hence 

when  the  terminals  of  the 
closed  or  secondary  cir- 
cuit of  a  receiving  tuner 
are  connected  to  the  nega- 
tive side  of  the  filament 
and  to  the  plate  of  the 
valve,  the  alternating  cur- 
rent of  radio-frequency 
flowing  in  the  receiver 
circuits  will  be  converted 
into  a  uni-directional  cur- 
rent which  may  affect  a 
head  telephone  or  other 
recording  instrument. 
More  clearly,  the  oscilla- 
tions flowing  in  the  re- 


Fig.    161 — Fleming    Oscillation    Valve    and    Tuning    Circuits. 


ceiver  circuits  during  the  time  of  a  complete  group  will  be  rectified  by  the  valve  placing  a 
charge  in  the  condenser  C-2  (Fig.  161),  which  afterwards  discharges  through  the  head 
telephone,  creating  a  single  sound  for  each  group. 

The  adjustment  of  the  Fleming  oscillation  valve  i-s  extremely  simple  and  the  stability  of 
the  device  particularly  marked,  in  fact  it  is  only  necessary  to  adjust  the  incandescence  of 
the  filament  until  loud  response  from  a  given  station  is  obtained  in  the  head  telephones. 
This  is  accomplished  by  the  rheostat  R  which  normally  is  of  10  or  15  ohms  resistance. 


Fig:.    162— Fundamental   Circuit  of   Marconi   Valve  Tuner. 

The  Fleming  valve  gives  signals  of  greatest  strength  when  the  secondary 
circuit  is  designed  for  a  minimum  value  of  shunt  capacity  and  a  maximum  value 
of  inductance  for  a  given  wave  length  or  frequency,  hence  the  secondary  con- 
denser is  always  worked  at  low  values  of  capacity. 

Many  valves  are  more  sensitive  if  an  added  potential  is  applied  between  the  filament 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


143 


and  the  plate.  Like  the  carborundum  detector,  the  Fleming  valve  has  a  rising  char- 
acteristic and  when  an  E.  M.  F.  is  applied  to  the  space  between  the  plate  and  the  fila- 
ment the  current  may  be  adjusted  to  the  critical  point  on  the  characteristic  curve  so  that 
the  addition  of  a  slight  antenna  voltage  causes  a  large  increase  of  the  local  battery  current 
flowing  through  the  head  telephones. 

A  satisfactory  diagram  appears  in  Fig.  162  wherein  a  potentiometer  P-l  is 
shunted  across  the  filament  battery  and  a  portion  of  the  current  flows  through 
the  head  telephones  and  the  detector.  With  this  arrangement  of  circuits  ad- 
vantage is  taken  of  the  particular  volt-ampere  characteristic  of  the  valve  and  a 
relay  action  due  to  a  local  source  of  energy  is  obtained.  For  the  maximum 
strength  of  signals  with  this  circuit  it  is  necessary  to  adjust  the  position  of  the 
sliding  contact  on  the  potentiometer  as  well  as  the  incandescence  of  the  filament, 
until  the  desired  results  are  obtained. 

The  diagram,  Fig.  162,  shows  the  fundamental  circuit  of  the  widely  used 
Marconi-Fleming  valve  receiver.  The  open  circuit  comprises : 

(1)  The  aerial  tuning  inductance  L-l, 

(2)  The  short  wave  condenser  C-l, 

(3)  The  primary  winding  L-2, 

(4)  The  shunt  impedance  R-l, 

(5)  The  change-over  switch  S-l,  S-2,  S-3. 

The  intermediate  circuit  comprises : 

(1)  The  winding  L-3  in  inductive  relation  to  L-2, 

(2)  The  winding  L-4  in  inductive  relation  to  L-5, 

(3)  The  variable  condenser  C-2. 

The  secondary  circuit  comprises : 

(1)  The  secondary  winding  L-5, 

(2)  The  billi  condenser  C-3, 

(3)  The  Fleming  valve  F,  P, 

(4)  The  battery  B  (or  four  volts  generally), 

(5)  The  10  ohm  rheostat  R. 

(6)  The  400  ohm  potentiometer  P-l, 

(7)  The  fixed  condenser  C-4, 

(8)  The  head  telephone  P-l. 

Inductances  L-2,  L-3,  L-4,  L-5,  L-6  are  of  fixed  value,  but  L-l  is  variable  through 
the  medium  of  a  multi-point  switch. 

When  the  D.  T.  three  blade  switch  S-l  is 
thrown  to  the  right,  primary  winding  L-2  is 
connected  in  series  with  the  aerial  circuit 
which  brings  the  intermediate  circuit  into  use, 
but  the  switch,  thrown  in  the  opposite  direc- 
tion, disconnects  L-2  and  connects  L-6  in 
series  with  the  aerial.  L-6  being  wound 
tightly  about  the  turns  of  L-5,  the  open  and 
closed  circuits  are  closely  coupled.  This  in- 
creases the  damping  of  the  receiving  system 
and  makes  the  set  responsive  to  waves  of  dif- 
ferent length  at  one  set  of  adjustments,  but, 
of  course,  does  not  give  the  strength  of  sig- 
nals that  can  be  obtained  by  resonant  adjust- 
ments. 

The  position  of  the  switch  corresponding 
to  "close  coupling"  is  marked  "Stdbi,"  an  ab- 
breviation for  the  word  "standby."  The  switch 
is  placed  in  this  position  when  a  particular 
receiving  station  awaits  a  call  from  one  of 
several  sending  stations  which  are  not  exactly 
tuned  to  the  same  wave  length. 

In    the    opposite    position,   the    changeover 

...  >,         1      -^     L\         •        -a.         Fig.     1&3 — Carborundum    Crystal    Holder    to    Fit 

switch  is  marked     tune    and  with  the  circuits  Fleming  Valve  Socket 


CRY5TAL 


TENSION 
ADJU5TOR 


CONTACT 
POINT 


VALVE 
SOCKET 


144 


PRACTICAL  WIRELESS  TELEGRAPHY. 


of  this  connection,  sharp  resonant  adjustments  can  be  obtained.  The  wave  length  of  the 
intermediate  circuit  is  increased  or  decreased  by  the  variable  condenser  C-2  only  and  simi- 
larly the  wave  length  of  the  secondary  or  detector  circuit,  by  the  condenser  C-3. 

The  coupling  between  the  inductances  of  the  intermediate  circuit  and  inductively  related 
coils  is  varied  simultaneously  by  means  of  a  shaft  (and  knob)  upon  which  both  coils,  L-3 
and  L-4,  are  mounted. 

The  impedance  coil  R-l  prevents  the  accumulation  of  high  voltages  upon  the  dielectric 
of  the  variable  condenser,  which  may  be  punctured  in  case  the  windings  of  the  tuner  should 
come  in  direct  contact  with  the  high  voltage  wires  of  the  transmitting  apparatus. 

The  American  Marconi  Company  has  developed  a  crystal  holder  to  fit  in  the 
Fleming  valve  socket  as  shown  in  Fig.  163.  The  circuit  for  the  crystal  detector 
is  practically  identical  with  that  of  the  oscillation  valve  with  the  exception  that 
the  lighting  battery  is  connected  to  the  potentiometer  instead  of  to  the  lamp 
filament. 


-  ELEMENTAL Y- 

-  DIAGRAM   OF  CONNECTIONS  — 

-  107-A    TUNER  - 


EXTERNAL  CRYSTAL 


Fig.    164 — Complete    Circuits    of    Type    107a    Tuner    (American    Marconi    Company). 

The  valve  tuner  was  original  designed  to  be  responsive  to  waves  varying  be- 
tween 300  and  1 ,650  meters  in  length,  but  the  circuit  has  been  modified  to  respond 
to  waves  up  to  2,500  meters,  as  in  Fig.  164. 

136.  Marconi  Type  107-a  Tuner.  (American  Marconi  Company). — The 
various  connections  and  arrangements  of  circuits  of  the  Marconi  type  107-a 
tuner  are  shown  in  Figs.  165-a,  165-b  and  165-c.  The  principal  change  over  the 
ordinary  valve  circuit  lies  in  the  special  six  point  double  throw  switch  (marked 
"change-over  switch")  which  disconnects  the  condenser  of  large  capacity  (.01 
microfarads)  across  the  intermediate  circuit  and  places  it  in  shunt  to  the  second- 
ary winding  and  the  billi- condenser  (see  Fig.  165-a).  With  the  increased  capacity 
of  this  condenser  in  shunt,  the  secondary  circuit  will  respond  to  a  wider  range 
of  wave  lengths  than  the  original  design  of  the  tuner  would  permit. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


145 


For  the  reception  of  wave  lengths  exceeding  1,500  meters,  the  D.  P.  D.  T. 
switch  is  thrown  to  the  "standby**  position  and  the  intermediate  circuit  cut  out. 
For  wave  lengths  below  1,000  meters,  either  the  "standby"  or  "tune"  circuits  can 
be  employed  at  the  discretion  of  the  operator. 

It  is  to  be  noted,  although  the  107-a  tuner  is  designed  for  the  carborundum 
detector,  an  extra  set  of  posts  is  provided  for  an  additional  crystal  such  as 


Fig.    165a — "Standby"    Long   Wave   Adjustment    of   Marconi    Type    107a    Tuner. 

cerusite.  When  the  latter  detector  is  employed,  the  potentiometer  is  out  of  the 
circuit.  Observance  next  should  be  made  of  the  binding  posts  from  which  wires 
are  connected  to  the  receiver  circuits  and  to  the  contacts  of  an  aerial  change-over 
switch.  The  latter,  when  open,  protect  the  detector  and  head  telephones  from  the 
induced  potentials  of  the  local  transmitter. 


Fig.   165b — "Tune"  Circuits  of  Type  107a  Tuner. 

In  the  diagram,  Fig.  165-b,  L-l  is  the  aerial  tuning  inductance,  the  value  of  which  is 
altered  by  means  of  a  multi-point  switch  mounted  on  the  left  hand  front  of  the  tuner. 

C-l  is  the  short  wave  variable  condenser  connected  in  series  with  the  aerial  system.  In 
the  full  scale  position  it  short  circuits  itself  and  is  thus  cut  out  of  the  antenna  circuit.  This 
condenser  is  mounted  on  the  left  hand  side  on  the  top  of  the  tuner. 

L-2  is  the  primary  winding  of  the  receiving  tuner  and  has  a  fixed  value  of  inductance 
(not  variable). 


146 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Coil  L-3,  condenser  C-2,  coil  L-4  comprise  the  intermediate  circuit.  L-3  is  of  fixed  value 
and  is  in  inductive  relation  to  L-2.  L-4  has  the  same  dimensions  as  L-3,  but  it  is  in  in- 
ductive relation  to  coil  L-5. 

The  wave  length  of  this  circuit  is  varied  by  means  of  a  condenser  C-2. 


i-S 


C-J 


C-4 


Fig.    165c— "Standby"    Short   Wave   Circuits   of   107a    Tuner. 

Coils  L-3  and  L-4  are  wound  on  balls  or  spheres,  mounted  on  a  shaft  so  they  can  be 
turned  at  a  right  angle  to  L-2  and  L-5,  simultaneously.  This  is  effected  by  means  of  the 
coupling  knob  mounted  on  the  right  hand  end  of  the  tuner,  and  thus  the  coupling  between 
the  intermediate  circuit  and  the  antenna  and  detector  circuits  is  varied  as  required. 


&U.U 

*'  CONDENS 


Fig.   166— Type   107a  Tuner   (Modified  Valve  Tuner). 

Coil  L-5  and  condenser  C-3  constitute  the  secondary  circuit.     L-5  is  of  fixed  value  and 
C-3  is  the  well  known  "billi"  condenser,  having  a  value  of  about  .0001  microfarad. 
C-4  is  the  telephone  condenser  of  approximately  .003  microfarad  capacity. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS.  147 

R  is  a  sliding  contact  potentiometer  of  400  ohms,  mounted  on  the  left  hand  side  of  the 
tuning  box. 

B  is  a  dry  cell  battery  of  1.5  to  3  volts,  while  D  is  the  usual  carborundum  crystal. 

The  inductance  coil,  L-7,  is  an  inductive  static  leak  for  preventing  the  accumulation  of 
large  static  charges  upon  the  plates  of  the  short  wave  variable  condenser  to  prevent  the 
rubber  dielectric  from  puncturing. 

During  adjustment  of  the  carborundum  crystal,  the  connection  from  the  battery  to  the 
potentiometer  must  be  reversed  until  the  best  response  is  obtained. 

The  intermediate  circuit  of  either  of  the  foregoing  tuners  is  employed  in  case  of  ex- 
cessive interference  from  nearby  stations.  Of  course  it  is  difficult  to  distinguish  between 
stations  operating  on  the  same  wave  length  except  as  the  signals  from  the  desired  station 
are  very  much  louder  than  those  from  the  interfering  station ;  but  transmitters  operating  on 
the  same  wave  length  and  having  different  degrees  of  damping  can  often  be  tuned  out  or 
in  by  proper  adjustment  of  the  coupling. 

It  will  be  apparent  from  the  preceding  diagrams  that  either  the  valve  or  the  107a  tuner 
is  tuned  to  resonance  with  the  incoming  signal  by  condensers  only,  with  the  exception  of 
the     aerial     tuning     inductance     which     is 
variable    in    steps    through    a    multipoint 
switch.      A  photograph   of  the    107a    tuner 
appears    in    Fig.    166.      In    adjusting    this 
tuner,    the    following    general    instructions 
should  be  observed: 

(a)  Operation.       With    the    connection 
("Tune")    indicated  in   Fig.   165b  the  tun- 
ing  is    extremely   sharp    and    the    circuits 
will  not  respond  to  wave  lengths  in  excess 
of  1,000  meters. 

For  general  work  the  coupling  knob  is 
set  at  90°.  The  billi  condenser  is  set  at 
the  zero  position  on  the  scale  and  one  or 
two  points  of  the  aerial  tuning  inductance 
connected  in  the  circuit.  The  setting  of 
the  intermediate  condenser,  C-2,  is  altered 
until  response  is  secured. 

Correct  setting  for  the  potentiometer  is  Fig    166a_Marconi  Disc  Variable  Condenser. 

obtained  by  means   of  a   buzzer  tester  or 

by  listening  to  the  signals  of  a  distant  station.     Follow  this  by  reversing  the  connections 
from  the  battery  to  the  tuner  until  the  loudest  signals  are  obtained. 

For  tuning  to  waves  up  to  1,000  meters,  three  or  four  points  of  inductance  should  be 
added  at  the  aerial  tuning  inductance,  but  for  the  wave  lengths  shorter  than  600  meters,  the 
short  wave  condenser  should  be  used  at  smaller  values  of  capacity. 

The  billi  condenser  permits  the  detector  circuit  to  be  tuned  to  waves  from  450  meters  to 
1,000  meters  in  length. 

(b)  "Stand-by"  Position.     When  the  double  throw  knife  blade  switch  is  thrown  to  the 
left — "stand-by"  position — the  circuits  are  altered  as  in  Fig.  165c.     With  this  connection  the 
primary  winding  L-2  is  disconnected  from  the  antenna  circuit  and  a  second  primary  wind- 
ing, L-6  connected  instead. 

L-6  is  an  inductance  of  fixed  value  wound  tightly  around  the  winding  L-5,  giving  a 
close  degree  of  coupling  between  the  aerial  and  detector  circiuts. 

From  the  diagram  it  will  be  plain  that  coil  L-4  of  the  intermediate  circuit  is  still  in 
inductive  relation  to  L-5  and  unless  the  precaution  is  taken  to  turn  the  coupling  knob  to  the 
zero  position,  considerable  energy  will  be  absorbed  from  the  detector  circuit,  thereby  re- 
ducing the  strength  of  the  incoming  signals,  particularly  if  the  intermediate  circuit  is  in 
resonance  with  the  antenna  and  the  detector  circuits. 

For  tuning  to  waves  up  to  approximately  1,000  meters  with  the  "Stdbi"  connection,  only 
the  billi  condenser  is  employed  in  shunt  to  the  secondary  winding,  but  for  longer  wave 
lengths  the  small  six-point,  double  throw  switch,  mounted  on  the  top  of  the  tuner  (marked 
change-over  switch  in  Fig.  164),  is  placed  in  the  "Stdbi  long  wave"  position,  whereupon  the 
intermediate  condenser,  C-2,  is  connected  in  shunt  to  the  billi  condenser  C-3.  This  connec- 
tion permits  waves  in  excess  of  3,000  meters  to  be  adjusted  to  in  the  secondary  circuit. 


148  PRACTICAL   WIRELESS    TELEGRAPHY. 

The  complete  circuit  for  this  connection  is  indicated  in  Fig.  165a.  Note  carefully  that 
the  intermediate  circuit  is  not  employed.  It  is  important  to  note  also  that  the  type  107-a 
tuner  is  fitted  with  four  binding  posts  (at  the  rear)  from  which  connections  extend  to  the 
type  S  aerial  changeover  switch.  When  the  antenna  switch  is  placed  in  the  transmitting 
position,  the  circuits  of  the  107-a  tuner  are  interrupted  at  the  points  A  and  B  (Fig.  165b), 
thus  breaking  the  circuit  to  the  detector  and  the  head  telephones.  The  contacts  at  this 
switch  must  have  careful  inspection  from  time  to  time,  for,  unless  they  close  properly,  the 
apparatus  positively  will  not  function. 

Should  these  contacts  be  broken,  a  permanent  jumper  should  be  placed  across  the  bind- 
ing posts  to  keep  the  circuit  closed. 

Type  107-a  tuner  should  be  used  with  the  type  S  or  type  I  aerial  changeover  switches 
only. 

The  complete  circuits  for  the  tuner  are  shown  in  detail  in  Fig.  165a  and  with  the  fore- 
going explanation,  the  functions  of  the  various  elements  should  be  clear  without  further 
instruction. 

(c)  General  Instructions.  For  "stand-by"  tuning  or  broad  adjustment  at  wave  lengths 
up  to  1,000  meters  (see  Fig.  165c),  place  the  double  throw  knife  switch  to  the  left. 

Set  the  condenser  switch  on  "tune." 
Set  the  coupling  knob  at  zero.  Carefully 
adjust  the  billi  condenser.  Connect  in  a 
few  points  of  the  aerial  tuning  inductance. 
Vary  the  capacity  of  the  short  wave  con- 
denser. 

For  long  wave  lengths  (in  excess  of 
1,000  meters)  place  the  condenser  switch 
on  "stand-by  long  wave"  position.  Vary 
carefully  the  capacity  of  the  intermediate 
condenser.  Add  inductance  at  the  aerial 
tuning  inductance. 

For  sharp  tuning  on  the  shorter  wave 
lengths  (below  1,000  meters),  place  the 

double  throw  knife  switch  to  the  "tune"  position.  Place  the  condenser  switch  on  the  "tune" 
position.  Set  the  coupling  knob  at  from  70°  to  90°.  Adjust  carefully  the  intermediate  con- 
denser. Add  two  or  three  points  of  inductance  at  the  aerial  tuning  inductance.  Follow 
this  by  variation  of  the  capacity  of  the  short  wave  condenser.  In  this  position,  all  the 
variable  elements  of  the  complete  tuner  are  in  use. 

137.  Marconi  Magnetic  Detector  and  the  Multiple  Tuner  Circuits  (Eng- 
lish Marconi  Company). — The  magnetic  detector  shown  diagrammatically  in 
Fig.  167  is  an  oscillation  detector  of  unvarying  stability.  A  description  of  the 
detector  and  an  explanation  of  its  functioning  follow : 

A  continuous  band  made  up  of  a  group  of  fine  iron  wires  revolves  on  the  ebony 
grooved  wheels  W  which  are  turned  by  clockwork  in  the  base  of  the  instrument.  The 
band  passes  through  the  glass  tube  G,  which  has  a  small  single  layer  of  fine  wire  comprising 
6  to  10  turns  through  which  the  radio-frequent  oscillations  flow.  Directly  over  this  wind- 
ing is  placed  the  small  bobbin  of  wire  S  which  has  approximately  the  resistance  of  the 
head  telephones  P-l.  Two  horseshoe  permanent  magnets  with  like  poles  adjacent  are 
mounted  immediately  above  the  tube  and  near  to  it.  Some  argument  exists  concerning  the 
action  of  this  detector  during  the  reception  of  signals  but  it  is  sufficient  to  say  that  when 
the  iron  band  passes  underneath  the  two  permanent  magnets  it  undergoes  a  cyclic  change 
in  magnetism  and  is  extremely  sensitive  to  an  impressed  external  magnetic  field  such  as 
that  generated  by  a  radio-frequent  current  flowing  through  the  winding  G. 

When  electrical  oscillations  induced  in  a  receiver  aerial  by  a  distant  transmitter  pass 
through  the  winding  P,  one  complete  group  of  oscillations  sets  up  an  alternating  magnetic 
field  which  causes  a  single  movement  or  a  change  in  the  position  of  the  flux  in  the  iron 
band.  The  bobbin  S  being  in  the  path  of  the  flux,  it  is  acted  upon  inductively,  a  current 
being  induced  in  the  windings  which  flows  through  the  head  telephone  creating  a  single 
sound  for  each  group  of  incoming  oscillations.  The  note  of  the  transmitter  is  faithfully 
reproduced,  because  each  group  of  oscillations  radiated  by  the  transmitter  has  a  cumulative 
effect  on  the  change  of  flux  in  the  iron  band,  which  creates  a  single  movement  of  the  tele- 
phone diaphragm.  Although  the  magnetic  detector  lacks  the  sensitiveness  of  the  crystal 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


149 


detector  throughout  the  shorter  range  of  wave  lengths,  as  between  300  and  600  meters, 
for  waves  around  2,500  meters,  it  compares  favorably  with  the  most  sensitive  crystal 
rectifiers. 

The  magnetic  detector  may  be  connected  directly  in  series  with  the  aerial 

system,  but  it  func- 
tions best  in  the 
circuits  of  the  Mar- 
coni Multiple  Tun- 
er (English  Mar- 
coni Company)  in 
Figs.  168  and  169. 
In  general,  the 
circuits  of  this 
tuner  appear  simi- 
lar to  that  of  the 
valve  tuner,  but  the 
secondary  winding 
for  the  magnetic 
detector  has  very 

few  turns  as  compared  with  the  secondary  of  the  valve  tuner.  The  magnetic 
detector  has  low  resistance  compared  with  crystal  detectors  and,  in  consequence, 
the  stopping  condenser  of  the  usual  detector  circuit  becomes  an  active  element  of 
the  closed  oscillation  circuit  and  must  therefore  be  of  variable  capacity.  In  fact, 
placing  the  detector  in  series  with  the  secondary  circuit  not  only  calls  for  a  wind- 
ing of  low  inductance  but  one  of  low  resistance  as  well.  Hence  the  secondary  coil 
is  usually  wound  with  rather  coarse  wire,  such  as  No.  18  or  No.  20  B.  and  S. 


Fig.    167a — Marconi    Multiple   Tuner. 


C»-  C,  IN  SERIES 
A  =  80 TO  ISO  METEFW    Cs  •  Ct 
I   C*    C, 


C-  IN  SERIES  WITH  AERIAL 

C,  ACROSS    INTERMEDIATE   CIRCUIT 

C «  IN  SEWES  WITH  MAGNETIC  DETECTOR. 


Ibb  —  fundamental    Circuit    Marconi's   Multiple   Tuner. 


A  fundamental  wiring  diagram  of  the  Marconi  multiple  tuner  appears  in  Fig.  168  and  a 
more  detailed  diagram  showing  approximately  the  position  of  the  apparatus  in  the  tuner 
in  Fig.  169.  Like  notations  are  used  in  both  drawings. 

Since  this  tuner  is  adjustable  to  wave  lengths  between  80  and  approximately  3,000 
meters,  a  specially  constructed  series  of  multi-point  switches  S-l,  S-2  and  S-3  (Fig-  169) 
control  the  inductance  and  capacity  of  the  primary,  intermediate  and  secondary  circuits  for 
a  progressive  increase  or  decrease  in  wave  length.  The  particular  wave  length  adjustment 
corresponding  to  any  position  of  the  switch  is  clearly  marked  on  the  operating  handle. 


150 


PRACTICAL   WIRELESS   TELEGRAPHY. 


When  the  small  D.  P.  D.  T.  switch  is  placed  in  the  "standby"  position,  the  magnetic 
detector  is  connected  directly  in  series  with  the  aerial  and  the  necessary  tuning  adjustments 
are  made  at  inductance  L-l  and  condenser  C-l,  but  when  this  switch  is  thrown  to  the 
'"tune"  position,  the  aerial  circuit  continues  through  the  primary  winding  L-2,  the  oscilla- 
tions being  transferred  from  thereon  to  the  intermediate  coil  L-3,  built  up  in  amplitude  by 
condenser  C-2  and  finally  transferred  to  L-5  by  induction  from  L-4. 

The  function  of  the  switches  S-l,  S-2  and  S-3  will  be  clear  from  the  drawings  and  the 
following  explanation,  describing  the  connections  of  the  various  fixed  and  variable  capaci- 
ties for  the  complete  range  of  wave  lengths. 


MAGNETIC  DETECTOR 


Fig.    169 — Detailed  Wiring  Diagram  of  Multiple  Tuner. 


For  wave  lengths  from  80 
to  150  meters. 


For  wave  lengths  from  150 
to  600  meters. 


For     wave     lengths     from 
1,600  to  2,000  meters. 


For     wave     lengths     from 
2,000  to  2,600  meters. 


Condensers  C-4  and  C-l  are  in  series. 
Condensers  C-5  and  C-2  are  in  series. 
Condensers  C-6  and  C-3  are  in  series. 

Condenser  C-l  is  in  series  with  the  antenna. 
Condenser  C-2  is  in  shunt  to  the  intermediate  circuit. 
Condenser  C-3  is  in  series  with  the  magnetic  detector. 

Condenser  C-l  may  be  in  or  out  of  the  aerial  circuit  as 

required. 

Condenser  C-7  is  in  shunt  to  condenser  C-2. 
Condenser  C-8  is  in  shunt  to  condenser  C-3. 
Condenser  C-l  is  in  or  out  as  required. 
Condensers  C-9  and  C-7  are  in  shunt  to  condenser  C-2. 
Condensers  C-10  and  C-8  are  in  shunt  to  condenser  C-3. 


The  switch  S-l  in  the  foregoing  drawings  selects  such  values  of  inductance  at  L-2  as 
will  give  the  correct  degree  of  coupling  at  all  wave  lengths.  The  protective  leak  inductance 
R  prevents  the  accumulation  of  heavy  charges  in  the  condenser  C-l.  The  coupling  be- 
tween L-2  and  L-3,  and  L-5  and  L-4,  is  varied  simultaneously  by  a  single  knob  on  the  side 
of  the  box.  A  protective  gap  R  protects  the  primary  circuit  of  the  tuner  should  by  accident 
the  circuits  come  in  contact  with  the  high  voltage  wires  .of  the  transmitter. 

Because  of  the  small  secondary  winding,  this  tuner  is  not  suitable  in  the  "tune"  position 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


151 


for  a  crystal  detector,  but  will  give  some  response  on  the  "standby"  side  in  which  connec- 
tion the  crystal  is  connected  in  series  with  the  aerial. 

It  has  frequently  been  observed  that  the  magnetic  detector  gives  better  response  to  500 
cycle  transmitters  when  the  iron  band  revolves  at  a  speed  about  twice  that  used  in  receiving 
from  60  cycle  transmitters.  Normally  the  band  travels  at  a  very  low  speed — a  few  feet  per 
minute. 


Fig.  170 — Type   106  Receiving. Tuner  (American  Marconi  Company,). 

138.  The  Marconi  Type  106  Receiving  Tuner.  (American  Marconi  Com- 
pany).— A  receiving  set  of  particular  excellence  combining  mechanical  and 
electrical  features  of  merit  is  the  type  106  tuner  of  the  American  Marconi  Com- 
pany. This  set  is  of  the  panel  type,  the  necessary  controlling  switches  being 
mounted  on  the  front  of  the  board  as  shown  in  photograph  170,  the  coupler, 
variable  condensers,  potentiometer,  etc.,  being  mounted  on  the  rear  as  in  photo- 
graph 171.  The  coupling  between  the  primary  and  secondary  windings  is  varied 
by  means  of  a  special  rack  and  pinion  adjustment  which  is  in  turn  controlled  by 
a  knob  on  the  front  of  the  panel  with  a  scale  marked  from  zero  to  10  (marked 
"coupling"). 

The  inductance  of  the  primary  winding  is  changed  by  means  of  two  multi- 
point switches,  one  of  which  operating  in  conjunction  with  a  barrel  switch  discon- 
nects the  unused  portions  of  the  primary  winding  for  any  particular  adjustment 
of  wave  length.  That  is,  the  primary  tvinding  is  divided  into  four  groups  which 
are  cut  into  the  circuit  as  the  wave  length  is  progressively  increased.  The  con- 
nections to  the  separate  aerial  tuning  inductance  connected  in  series  with  the 
antenna  system  are  included  in  the  "tens"  switch  of  the  primary  winding.  The 
"unit"  switch  controls  10  single  turns  of  the  primary  winding  permitting  any 
number  of  turns  from  one  to  maximum  to  be  included  in  the  circuit  (as  will  be 
explained  in  paragraph  191). 

The  aerial  circuit  includes  a  short  wave  variable  condenser  which  in  the  180° 
position  short  circuits  itself  automatically  by  means  of  special  contacts  fitted  to 
the  movable  and  stationary  plates.  The  secondary  coil  divided  into  10  groups 


152 


PRACTICAL   WIRELESS   TELEGRAPHY. 


is  fitted  with  an  end-turn  switch  which  splits  the  winding  into  three  complete 
groups.  The  secondary  winding  is  shunted  by  a  variable  condenser  which  in  the 
zero  position  is  completely  disconnected  from  the  circuit,  thereby  cutting  off  the 


Fig.  171—  Rear  View  Type  106  Receiving  Tuner. 

capacity  effect  between  the  opposite  plates   (of  the  variable  condenser)   in  the 
so-called  zero  position  of  capacity. 

A  special  wire  wound  potentiometer  is  supplied,  having  a  resistance  of  about 
450  ohms,  the  value  of  which  is  adjusted  by  a  rotary  multipoint  switch. 

The  receiving  de- 
lector,  placed  in  a  ver- 


SHORT  WAVE  VARIABLE 


T 


TESTER 
1/2 — -Fundamental   Circuit    Diagiam    106   Tuner. 


front  of  the  panel, 
consists  of  a  contact 
point  with  spring- 
pressure  mounted  on 
a  universal  joint 
which  may  be  placed 
in  contact  with  the 
sensitive  spot  on  one 
of  several  crystals 
mounted  in  a  cup  di- 
rectly underneath. 

A  fundamental  dia- 
gram of  the  type  106 
tuner  is  shown  in  Fig. 
172,  which,  as  will  be 
observed,  is  somewhat 
similar  to  the  connec- 
tions of  Fig.  1531x 
The  part  of  this  dia- 


ticular  attention  is  the 
connection      of      the 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS.  153 

potentiometer,  the  head  telephones  and  the  fixed  condenser  C-3,  and  the  group- 
ing of  the  primary  and  secondary  turns.  The  buzzer  excitation  system  shown 
will  be  described  in  detail  in  paragraph  149. 

The  reader  should  note  that  the  106  tuner  is  constructed  so  that  only  the  used  turns  of 
the  primary  and  secondary  windings  are  in  direct  inductive  relation,  that  is,  the  unused 
turns  of  either  winding  do  not  overlap  and  consequently  when  the  primary  and  secondary 
are  set  at  low  values  of  inductance,  care  must  be  taken  to  have  these  two  windings  in  the 
correct  inductive  relation.  For  instance,  by  a  little  consideration,  it  will  be  evident  that  if 
the  secondary  winding  is  placed  too  far  inside  the  primary  coil  the  coupling  is  decreased  just 
as  it  is  when  the  secondary  winding  is  drawn  out  of  the  coil. 

The  general  instructions  for  the  adjustment  of  the  inductive  receiving  tuner  given  in 
paragraph  130  are  thoroughly  applicable  to  the  type  106  tuner.  An  increase  of  inductance 
in  the  antenna  circuit  for  adjustment  to  a  given  wave  length  should  be  followed  by  an 
increase  of  the  secondary  inductance  to  maintain  resonance.  Also  with  each  change  of 
wave  length  there  is  a  critical  degree  of  coupling  which  gives  the  louder  signals  in  the 
head  telephone. 

Under  conditions  of  loose  coupling  the  secondary  variable  condenser  is  of  some  use 
to  reduce  interference,  but  under  conditions  of  close  coupling  the  condenser  is  preferably 
set  near  to  zero  values  of  capacity. 

The  short  wave  variable  condenser  is  employed  to  tune  the  antenna  circuit  to  signals  the 
wave  length  of  which  is  below  the  natural  wave  length  of  the  receiving  antenna.  Response 
to  shorter  waves  is  obtained  at  capacities  near  to  the  zero  position  of  the  condenser  scale. 
With  this  condenser  in  circuit,  the  wave  length  of  the  antenna  system  will  be  progressively 
increased  (that  is,  the  oscillation  frequency  decreased)  as  the  control  handle  is  moved  in  the 
direction  of  maximum  capacity.  In  case  of  severe  interference,  the  damping  of  the  receiv- 
ing antenna  may  be  decreased  in  the  following  manner:  For  a  given  wave  length  set  the 
short  wave  variable  condenser  at  a  certain  value  of  capacity  and  obtain  resonance  by 
adding  inductance  at  the  aerial  tuning  inductance  until  maximum '  response  is  obtained. 
Care  must  be  taken  to  select  the  correct  values  of  inductance  and  capacity,  which,  of 
course,  vary  with  each  aerial.  If  excessive  values  of  inductance  are  added,  variation  of 
capacity  will  have  little  effect  on  the  incoming  signals.  The  fact  is  that  signals  would  not 
be  heard  under  these  conditions  unless  the  transmitting  station  is  near  to  the  receiving 
station.  Correct  values  of  inductance  and  capacity  are  assured  if  a  slight  change  in  con- 
denser capacity  eliminates  the  signals. 


G 

Fig.     173 — Type     101    Receiving    Tuner     (American    Marconi    Company). 

We  may  resume  the  function  of  the  elements  of  the  tuner  as  follows:  The  small  push 
button  marked  "Test"  immediately  to  the  right  hand  side  of  the  receiving  tuner  closes  the 
circuit  from  the  battery  to  the  buzzer  which  permits  the  manipulator  to  obtain  the  best 
adjustment  of  the  crystal.  When  turned  to  the  right  this  button  is  locked  in  position.  The 
two  switches  on  the  front  of  the  Type  106  tuner  marked  "transformer  primary"  vary  the 
inductance  of  the  primary  circuit.  The  switch  marked  "Units"  cuts  in  a  single  turn  at  a 
time.  The  switch  marked  "Tens"  varies  the  primary  inductance  in  groups  of  ten  turns  at 
each  point  of  contact. 


154  PRACTICAL  WIRELESS   TELEGRAPHY. 

The  primary  condenser  has  minimum  capacity  in  the  zero  position  but  if  turned  to  the 
180°  mark  it  is  shunted  by  a  special  set  of  contacts  (attached  to  the  movable  plates  and 
thus  cut  out  of  the  aerial  circuit.  All  control  handles  attached  to  the  inductance  changing 
switches  and  variable  condensers  of  the  primary  and  secondary  circuits  should  be  turned 
counter  clockwise  for  increase  of  their  values. 


r 

Fig.    174 — Rear   View   Type    101    Receiving   Tuner    (American   Marconi    Company). 

The  switch  marked  "transformer  secondary"  and  the  control  handle  marked  "secondary 
condenser"  control  the  inductance  and  capacity  of  the  secondary  circuit.  The  wave  length 
adjustment  of  the  circuit  can  be  increased  by  turning  the  switch  or  the  knob  of  the  con- 
denser counter  clockwise.  Different  values  of  inductance  and  capacity  can  be  employed  in 
this  circuit  while  holding  the  same  wave  length.  This  changes  the  damping  of  the  circuit 
and  proper  proportioning  (of  inductance  and  capacity)  may  give  increased  strength  of 
signals  during  the  reception  of  signals  from  certain  stations. 

The  control  handle  marked  "potentiometer"  varies  the  flow  of  current  through  the  de- 
tector crystal.  Close  adjustment  of  the  potentiometer  is  necessary  when  the  incoming 
signals  are  comparatively  weak.  The  switch  marked  "battery"  turns  the  local  battery  cur- 
rent on  and  off. 

The  terminals  marked  "battery"  connect  to  four  dry  cells  in  a  separate  box  and  the 
terminals  marked  "telephones"  are  the  binding  posts  for  connection  to  2,000  ohm  receivers. 

As  the  transmitter  operates  at  approximately  the  wave  length  to  which  the  secondary 
circuit  of  the  receiver  is  adjusted,  it  produces  exceedingly  strong  signals  which  are  liable 
to  impair  the  sensitive  condition  of  the  crystal.  To  obviate  this,  connections  are  made  with 
the  antenna  switch  so  that  when  the  antenna  switch  is  in  the  transmitting  position  the 
terminals  of  the  detector  and  the  secondary  condenser  are  short-circuited.  Care  should  be 
taken  to  see  that  the  antenna  switch,  with  which  this  receiver  is  to  be  used,  is  so  con- 
structed as  to  perform  the  above  operations.  If  this  is  done,  the  transmitter  has  very  little 
or  no  effect  on  the  sensitiveness  of  the  crystal  and  it  will,  therefore,  be  in  a  sensitive 
condition  for  receiving  immediately  after  transmission.  (See  diagrams,  Figs.  201,  202 
and  203.)  In  addition  to  paragraph  130  the  student  should  read  the  general  instructions 
in  paragraph  156a. 

139.  Marconi  Receiving  Tuner  Type  101  (American  Marconi  Company). — 
The  type  101  receiver  consists  of  an  inductively  coupled  transformer  with  two 
solid  rectifier  detectors  and  the  necessary  accessory  apparatus  mounted  on  a  hard 
rubber  panel  and  enclosed  in  a  mahogany  case.  The  front  elevation  of  this  set 
is  shown  in  photograph,  Fig.  173,  the  rear  elevation  in  Fig.  174,  and  a  funda- 
mental diagram  in  Fig.  175. 

The  aerial  is  connected  to  the  binding  post  "antenna"  and  the  earth  connection  is  made 
to  the  binding  post  marked  "ground."  The  circuit  between  these  two  points  is  adjusted  to 
resonance  with  the  incoming  signal  by  variation  of  the  two  transformer  primary  switches, 
the  primary  loading  coil  switch,  the  primary  condenser,  and  the  primary  condenser  switch 
which  is  marked  "out"  "series"  and  "shunt."  The  purpose  of  this  switch  is  to  connect  the 
primary  condenser  in  series  or  in  parallel  to  the  aerial  or  to  disconnect  the  condenser 
entirely. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


155 


Thus  for  long  wave  lengths  the  condenser  is  placed  in  shunt  to  the  primary  winding 
and  for  waves  below  the  fundamental  period  of  the  aerial  the  condenser  is  thrown  in 
series  with  the  primary  winding. 

The  function  of  the  three  other  primary  switches  is  to  vary  the  number  of  turns  of 
inductance  in  the  aerial  circuit. 

The  secondary  circuit  contains  the  transformer  secondary  coil,  the  inductance  of  which 
is  varied  by  the  transformer  secondary  switch,  the  secondary  condenser,  the*  cerusite  and 
the  carborundum  detectors,  the  switch  for  connecting  either  of  these  two  in  the  circuit, 
the  potentiometer,  and  the  switch  marked  "sent,"  "receive." 


ANTENA 
B 


175 — Fundamental  Diagram  Type  101  Tuner. 


The  transformer  secondary  is  moved  in  or  out  of  the  transformer  primary  coil  by 
means  of  the  handle  marked  "coupling."  The  capacity  of  the  secondary  condenser  is 
varied  by  rotation  of  its  handle,  and  the  potential  across  the  carborundum  detector  is 
varied  by  the  rotation  of  the  potentiometer  handle. 

Supplied  with  this  set  is  a  mahogany  box  containing  four  dry  cells  having  three  binding 
posts,  marked  1,  2,  and  3.  Nos.  1  and  2  connect  to  the  two  binding  posts  on  the  receiver 
immediately  under  the  word  "battery,"  while  No.  3  is  connected  to  the  post  at  the  extreme 
right  on  the  same  level  with  "test"  switch.  This  post  completes  the  circuit  from  the  battery 
to  the  test  tuner. 

Since  the  detectors  work  better  with  the  battery  current  flowing  in  a  certain  direction, 
it  is  necessary  to  determine  experimentally  which  binding  post  to  connect  to  the  carbon 
and  which  to  the  zinc  electrode. 

The  head  telephones  are  connected  to  the  binding  post  "telephones."  The  binding  posts 
marked  "antenna  switch  A,"  posts  1  and  2,  and  "antenna  switch  B  and  C,"  posts  1,  2,  and  3, 
are  for  connection  to  a  specially  designed  navy  type  antenna  switch. 

When  the  navy  switch  is  not  used,  posts  Al  and  A2,  B2  and  B3,  Cl  and  C2,  and  B3  and 
Bl  are  short  circuited  with  pieces  of  thick  copper  wire. 

The  posts  marked  "extra  detector"  are  for  connection  with  an  outside  detector.  If  the 
outside  detector  requires  battery,  place  the  detector  switch  in  the  position  marked  "car- 
borundum," if  not  place  it  in  position  marked  "cerusite." 

The  test  buzzer  is  mounted  within  the  metal  cup  directly  over  the  secondary  condenser 
and  can  be  adjusted  if  necessary  by  removing  the  two  cap  screws. 

Since  the  satisfactory  operation  of  the  set  depends  to  a  great  extent  on  the  switch 
blades  making  perfect  connection  with  the  switch  studs,  it  is  necessary  to  see  that  there 
is  always  good  firm  pressure  between  them.  If  for  any  reason  this  pressure  becomes  too 
weak,  remove  the  screw  in  the  handle,  take  off  the  handle,  and  remove  the  two  screws 


156 


PRACTICAL    WIRELESS    TELEGRAPHY. 


holding  the  switch  blades  in  place.  The  blades  can  then  be  bent  slightly  so  that  when 
replaced  they  will  make  positive  contact. 

The  complete  adjustment  of  this  tuner  is  as  follows:  Set  the  primary  condenser  switch 
to  position  "Out,"  place  coupling  pointer  at  about  7  on  the  scale,  detector  switch  to  "ceru- 
site,"  secondary  condenser  pointer  to  position  "Out,"  potentiometer  to  O,  "send"  and  "re- 
ceive" switch  to  "receive"  (if  used  with  navy  switch  or  break  system  relay,  leave  in  "send" 
position  always),  turn  test  switch  to  "In"  position,  which  starts  the  buzzer;  then  adjust 
Cerusite  Detector  point  on  crystal  surface  until  loudest  response  is  heard.  This  crystal 
requires  very  light  pressure  for  maximum  sensitiveness  and  the  point  may  be  screwed  up 
or  down  by  turning  the  hard  rubber  knob  to  left  or  right. 

Having  found  a  sensitive  point  in  the  crystal,  vary  the  inductance  of  the  two  primary 


Fig.    176 — Wiring    Diagram    Marconi    Universal    Receiver. 


transformer  switches  until  the  desired  signal  is  heard,  then  loosen  the  coupling  by  rotating 
coupling  handle  to  the  right  until  the  signal  is  just  audible,  then  try  other  points  of  trans- 
former secondary  switch  and  rotate  secondary  condenser  handle  to  the  left  until  a  com- 
bination is  found  which  gives  maximum  response. 

The  primary  should  then  be  readjusted  more  accurately  until  the  best  setting  is  found 
and  the  coupling  then  adjusted  until  the  maximum  strength  of  signal  is  obtained.  The 
longer  the  wave  length  the  greater  the  number  of  turns  of  inductance  necessary  in  primary 
and  secondary  circuits.  If  more  inductance  is  necessary  for  a  particular  signal  and  cannot 
be  obtained  by  adjustment  of  transformer  primary  switch,  place  this  in  position  marked 
"Out"  and  rotate  the  primary  loading  coil  switch  from  its  position  "Out"  until  sufficient 
turns  are  in  the  circuit. 

If  the  wave  length  of  the  incoming  signal  is  shorter  than  can  be  reached  with  primary 
condenser  switch  in  the  "Out"  position,  place  it  in  the  series  position,  set  the  ten  turn  trans- 
former primary  switch  at  zero,  the  unit  .turn  transformer  primary  switch  at  10  and  rotate 
primary  condenser  handle  until  maximum  response  is  obtained. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


157 


Placing  the  primary  condenser  switch  in  the  "Shunt"  position  increases  the  wave 
length  of  the  aerial  circuit  corresponding  to  any  given  setting  of  the  inductance  switches. 
This  may  be  done  at  any  time  instead  of  increasing  the  inductance.  Generally  speaking,  the 
method  of  varying  inductance  is  preferable  and  gives  a  louder  response,  but  in  exceptional 
cases  the  reverse  is  true  and  in  any  particular  case  a  trial  of  the  two  methods  can  be  made 
to  find  out  which  is  the  better.  The  best  secondary  setting  for  maximum  response  to  any 
particular  wave  length  is  that  of  maximum  inductance  and  minimum  capacity,  but  the 
greatest  selectivity  will  be  obtained  with  smaller  inductance  and  greater  capacity. 

When  the  carborundum  detector  is  in  use  it  will  be  necessary  to  adjust  the  potentiometer 
to  a  point  which  gives  the  greatest  sensitiveness. 

It  should  be  noted  that  when  using  the  Cerusite  detector  during  the  reception  of  very 
weak  signals  the  potentiometer  should  always  be  in  zero  position,  but  for  signals  of  mod- 
erate intensity  it  does  not  matter  particularly  what  position  this  may  happen  to  be  in.  In 
order  that  the  operator  may  be  able  to  measure  approximately  the  wave  length  of  the 
incoming  signals,  calibrations  of  the  secondary  are  given  with  each  tuner,  it  being  under- 
stood that  these  are  correct  only  when  the  secondary  is  coupled  very  loosely  to  the  primary. 
With  the  101  receiver  are  supplied  adjustable  head  telephones  which  are  tuned  to  the 
group  frequency  of  the  transmitter.  Also  the  electrical  circuit  through  the  telephones  and 
the  stopping  condenser  is  tuned  to  the  same  group  frequency. 

It  is  advisable  occasionally  to  insert  a  piece  of  paper  between  the  spark  points  of  the 
antenna  and  ground  binding  posts  and  telephone  binding  posts  to  make  sure  they  are  not 
short-circuited. 

140.  The  Marconi  Universal  Receiving  Set  (English  Marconi  Company).— 

The  complete  circuits 
of  the  Universal  Crys- 
tal Receiver  are  indi- 
cated in  the  diagram. 
Fig.  176,  and  a  photo- 
graph of  the  finished 
instrument  in  Fig. 
177,  the  tuner  having 
been  designed  for 
wave  lengths  between 
300  and  3,000  meters. 
The  circuits  are  ap- 
plicable to  all  types  of 
crystal  rectifiers,  par- 
ticularly those  of  car- 
borundum, z  i  n  c  i  t  e, 
bornite,  etc. 

The  usual  aerial  tun- 
ing inductance  is  represented  at  L-l,  the  short  wave  condenser  at  C-l,  and  the  primary 
winding  of  the  tuning  transformer  at  L-2.  L-2  has  three  tappings,  which  are  varied  by 
means  of  the  switch.  The  secondary  circuit  of  the  receiving  tuner  is  divided  into  three 
units,  the  following  range  of  wave  lengths  being  obtained  with  each:  L-3  has  the  correct 
values  for  all  wave  lengths  up  to  600  meters;  L-4  for  waves  between  600  and  1,600  meters, 
and  L-5  for  waves  between  1,000  and  3,000  meters.  The  necessary  change  of  inductance  is 
obtained  by  the  wave  length  changing  switch  D,  which  completely  disconnects  the  unused 
turns  from  the  circuit. 

The  functioning  of  the  switch  is  as  follows :  In  one  position  the  switch  blade  D  is 
connected  to  the  high  potential  end  of  winding  L-5  through  contact  point  A ;  but  a  longer 
wave  length  adjustment  is  obtained  by  placing  the  switch  on  point  B,  which  connects  the 
receiving  detector  to  the  high  potential  end  of  L-4,  and  at  the  same  time  the  lower  right 
hand  contacts  alongside  the  switch  spring  together,  joining  the  high  potential  end  of  L-3 
to  the  low  potential  end  of  L-4.  The  same  function  is  performed  by  switch  point  C  and 
the  upper  right  hand  set  of  contacts. 

The  secondary  condenser  shown  at  C-3  possesses  at  maximum  very  low  capacity.  The 
potentiometer  P-2  is  shunted  across  the  battery  B-2.  A  protective  choke  is  indicated  at 
P-4  and  a  safety  gap  at  R.  The  telephone  condenser  C-2  is  of  fixed  capacity. 

A  novel  feature  of  this  tuner  is  the  magnets  M-l  and  M-2,  which  are  joined  to  battery 


Fig.     177 — Universal    Crystal    Receiver    (English    Mai 


>any). 


158 


PRACTICAL  WIRELESS   TELEGRAPHY. 


178— Showing 


Construction 
Detector. 


of      Electrolytic 


B-l,  the  terminals  X  and  X-l  extending  to. a  small  switch  alongside  the  transmitting  key. 
Closing  this  switch  energizes  the  magnets,  which  draw  the  contacts  on  either  side  of  the 
crystal  apart,  completely  disconnecting  all  attached  circuits  and  thereby  protecting  the 

detector  from  induction  of  the  transmitter. 
The  magnets  and  the  crystal  holder  are 
mounted  in  a  metallic  box. 

A  number  of  vessels  in  the  trans- 
Atlantic  service  of  the  Marconi  Com- 
pany are  fitted  with  this  apparatus 
which  has  been  found  to  permit  recep- 
tion over  very  long  distances. 

141.  Electrolytic  Detector.— A 
detector  widely  used  in  the  early  stages 
of  wireless  telegraph  development  in 
the  United  States  is  the  so-called 
"zvhisker  point"  electrolytic,  which  is 
particularly  sensitive  and  uniformly 
stable  in  operation.  Of  late,  however, 
the  electrolytic  cell  has  fallen  into 
almost  complete  disuse  for  commercial 
working,  even  though  its  reliability  is 
generally  admitted.  From  a  commer- 
cial standpoint  this  may  be  accounted  for  by  the  fact  that  the  initial  adjustment 
of  the  device  is  rather  troublesome. 

The  essentials  of  the  detector  appear  in  Fig.  178,  where  a  small  glass  receptacle  R  has 
sealed  in  the  base  a  small  piece  of  platinum  P  about  ^2-inch  square.  About  a  half-dozen 
drops  of  a  20  per  cent,  solution  of  nitric  acid  or  a  supersaturated  solution  of  caustic  potash 
cover  the  lower  electrode.  The  upper  electrode  P-l  is  an  extremely  fine  platinum  wire 
about  .0001  inches  in  diameter.  The  depth  of  immersion  in  the  liquid  is  carefully  regulated 
by  a  finely  threaded  screw  adjustment.  The  platinum  wire  is  generally  coated  with  silver, 
which  afterward  is  dissolved  by  dipping  the  point  of  the  wire  in  a  strong  solution  of  nitric 
acid,  leaving  the  small  platinum  tip  exposed. 

The  silver  need  not  necessarily  be  taken  off  the  platinum  wire  by  a  strong  acid  solution ; 
the  point  can  be  immersed  in  the  usual  detector  solution  of  dilute  nitric  acid  and  an  extra 
strong  local  current  sent  through  the  cell  for  a  few  moments  until  the  silver  is  completely 
dissolved.  Afterward  the  point  is  adjusted  to  just  touch  the  solution. 

Now,  if  this  detector  is  substituted  for  the  carborundum  rectifier  in  the  circuit  of  Fig. 
153a,  and  the  positive  pole  of  the  local  battery  connected  to  the  fine  wire  electrode,  the  de- 
vice becomes  a  very  sensitive  detector  of  currents  of  radio  frequency,  provided  the  small 
electrode  just  touches  the  surface  of  the  acid. 

The  fine  wire  electrode  is  frequently  coated  with  glass,  so  that  the  extreme  tip  only  is 
exposed  to  the  action  of  the  solution,  hence  the  depth  of  immersion  of  the  entire  electrode 
is  of  little  importance  and  the  detector  is  less  difficult  to  adjust. 

Several  theories  have  been  advanced  to  account  for  the  action  of  the  electrolytic  de- 
tector, one  being  that  the  response  in  the  local  head  telephone  is  caused  by  change  in  re- 
sistance of  the  small  platinum  wire  during  the  passage  of  radio-frequent  currents. 

Another  investigator  contends  that  the  current  of  the  local  battery  flowing  through  the 
electrolytic  cell  forms  gas  bubbles  which  polarize  the  fine  wire  electrode,  and  thereby  par- 
tially reduce  the  flow  of  current  from  the  local  battery.  Then  when  oscillations  of  radio- 
frequency  pass  through  the  cell,  the  gas  bubbles  are  temporarily  destroyed,  which  permits 
an  increase  of  the  strength  of  the  local  battery  current  at  rates  corresponding  to  the  spark 
frequency  of  the  transmitter. 

The  electrolytic  detector  is  adjusted  for  maximum  strength  of  signals,  by  carefully  regu- 
lating the  strength  of  the  local  battery  current.  If  the  current  is  too  strong,  a  hissing  sound 
is  obtained  in  the  head  telephone  which  will  prevent  the  reception  of  signals.  If,  on  the 
other  hand,  the  local  current  is  too  weak,  the  detector  will  barely  respond.  Some  difference 
of  opinion  exists  regarding  the  direction  in  which  the  local  current  must  flow  through  the 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


159 


cell,  but  it  is  usual  to  connect  the  fine  wire  electrode  to  the  positive  pole  of  the  cell. 
H.  Shoemaker  has  discovered  that  the  large  electrode  may  be  of  zinc,  and  if  the  small 
platinum  point  and  zinc  are  immersed  in  a  dilute  sulphuric  acid  solution,  the  cell  not  only 
acts  as  a  detector  of  oscillations,  but  supplies  its  own  local  E.  M.  F.  as  well.  In  this  form 
the  detector  is  termed  a  primary  cell  detector. 

142.  The  Three  Element  Valve  Detector.— A  modified  type  of  the  two 
element  Fleming  valve  is  the  three  element  valve,  the  circuits  for  which  are  shown 
in  Fig.  179.  From  the  viewpoint  of  mechanical  construction,  it  is  practically 
identical  with  the  Fleming  valve,  with  the  exception  that  a  metallic  element  known 
as  a  grid  is  placed  in  the  path  of  the  electrons  flowing  from  the  filament  to  the 
plate. 

This  grid  may  be  made  up  of  a  zig-zag  shaped  platinum  ^vire  or  a  plate  per- 
forated with  holes,  or  in  certain  types  of  valves  it  consists  of  a  spiral  of  copper. 
The  filament  may  be  constructed  of  tungsten,  tantulum  or  platinum. 
^ — •— -I  The  circuit  generally 

\      /  adopted    .for    the    three 

\/  element  valve  appears  in 

Fig.  179,  where  the  pri- 
mary and  secondary  cir- 
cuits of  a  receiving  tuner 
are  represented  by  the 
usual  notations  and  the 
grid,  plate  and  filament 
of  the  valve  by  G,  P,  and 
F,  respectively.  An  im- 
portant part  of  this  cir- 
cuit is  the  so-called  grid 
condenser  C-l,  which 
generally  has  low  capa- 
city, varying  between 
.00003  and  .0005  micro- 
farads. 

In  addition  to  the 
lighting  battery  B-l, 
which  is  generally  of  4 
volts,  a  local  battery 
B-2  of  35  to  200  volts 
has  its  positive  terminal 
connected  to  the  plate  P  and  the  negative  terminal  to  the  telephones  P-l,  which  are  in  turn 
attached  to  the  positive  end  of  the  battery  B-l.  The  secondary  terminals  of  the  receiving 
tuner  are  connected  to  the  negative  terminal  of  the  filament  and  to  the  grid. 

In  this  form  of  valve  as  in  that  already  described,  the  incandescent  filament  produces 
electrons  which  give  to  the  vacuous  space  conductivity  in  one  direction  only.  In  the  two 
element  bulb,  the  plate  element  serves  both  to  impress  the  received  oscillations  on  the 
vacuous  space  and  to  connect  the  telephone  thereto.  In  the  three  element  bulb  these  func- 
tions are  separated,  the  first  being  assigned  to  the  grid  and  the  second  to  the  plate.  A 
gain  in  flexibility  and  ease  of  manipulation  is  thereby  secured. 

As  in  the  two  element  valve,  the  electrons  carry  the  local  battery  current  as  well  as  the 
incoming  oscillations  and  the  effect  of  the  latter  is  to  vary  the  local  battery  current,  pro- 
ducing the  so-called  relay  action. 

According  to  the  explanation  advanced  by  Armstrong,  the  grid  clement,  if  placed  in  the 
path  of  the  electrons,  receives  a  negative  charge  which  decreases  the  local  battery  current, 
i.  e.,  the  current  flowing  between  the  plate  and  filament.  An  external  positive  charge  applied 
to  the  grid  will  in  a  degree  neutralize  the  negative  charge  and  thus  increase  the  local  bat- 
tery current,  but  an  external  negative  charge  will  cause  greater  absorption  of  electrons  and 
reduce  the  battery  current  to  a  lower  figure.  Hence,  if  an  alternating  E.  M.  F.  be  impressed 
between  the  grid  and  filament  the  positive  alternation  increases  the  local  battery  current 
and  the  negative  alternation  decreases  it. 

In  order  to  receive  the  maximum  strength  of  signals  from  spark  stations,  the  detector 
must  be  connected  in  a  circuit  that  will  permit  an  accumulative  effect  for  each  group  of 


Fig.     179 — Fundamental     Circuit 


for     the 
Valve. 


Three     Element       Oscillation 


160 


PRACTICAL   WIRELESS   TELEGRAPHY. 


POTENTIAL  Of 
GRID  TO 
FILAMENT 


WING 
CURRENT 


TELEPHONE 
CURRENT^ 


incoming  oscillations.  Such  a  circuit  is  shown  in  Fig.  179,  which,  according  to  Armstrong, 
functions  as  follows :  The  radio-frequent  oscillations  induced  in  the  secondary  circuit  of 
the  receiver  are  rectified  by  the  valve  action  between  the  grid  and  filament,  and  a  negative 
charge  is  placed  in  the  condenser  C-l  which,  at  the  termination  of  a  wave  train,  leaks  off 
the  grid,  exerting  a  relay  action  on  the  local  battery  current,  decreasing  its  strength  at 
audio-frequent  rates.  At  the  termination  of  a  group  of  incoming  oscillations,  the  grid 

returns  to  normal  poten- 
tial and  also  the  local 
current,  whereupon  the 
process  is  repeated.  If 
the  valve  is  properly  con- 
structed, i.  e.,  if  a  good 
sample  is  used,  it  is  a 
very  sensitive  detector, 
much  stronger  signals 
being  obtained  than 
with  an  ordinary  crystal 
rectifier. 

Oscillograms  of  this 
detector  taken  by  the 
same  investigator  indi- 
cate that  the  three  ele- 
ment valve  not  only  func- 
tions in  the  manner  just 
explained,  but  that  the 
oscillations  of  radio-fre- 
quency flowing  through 
the  grid  circuit  are  re- 
peated in  the  head  tele- 
phone circuit  and  super- 
imposed upon  the  local 

Fig.    180 — Curve  Showing  the   Process   by    which  the   Three   Element    Valve    u_ff._  TT 

makes    Damped    Oscillations    Audible.  battery     current.       How- 

ever,    the    radio-frequent 

oscillations  (repeated  in  the  local  battery  circuit)  do  not  affect  the  telephone  diaphragm, 
but  the  audio  frequency  variation  of  the  continuous  current  in  that  circuit  does  create  an 
audible  sound  of  a  pitch  determined  by  the  spark  frequency  of  the  transmitting  station. 

The  process  involved  in  the  detection  of  oscillations  induced  in  the  receiver 
by  a  distant  transmitting  station  can  be  shown  by  the  curves  of  Fig.  180,  which 
indicate  first :  the  oscillations  of  the  incoming  signals;  second,  the  potential  of 
the  grid  circuit  with  respect  to  the  filament;  third,  the  decrease  of  the  wing  cur- 
rent and  the  superposed  oscillations  of 
radio-frequency,  and  finally,  the  result- 
ing fluctuation  of  current  in  the  head 
telephone. 

Either  the  current  of  radio-  or 
audio-frequency  reproduced  in  the  lo- 
cal circuit  of  the  vacuum  valve  can  be 
increased  in  amplitude  by  special  con- 
nections on  the  same  valve  or  by  con- 
necting a  number  of  valves  in  cascade. 
In  the  first  method,  the  current  of 
radio-  or  audio-frequency  is  repeated 
back  to  the  grid  circuit  by  special 
radio-  or  audio-frequency  transform- 
ers, but  in  the  second  method,  a  trans- 
former is  connected  in  the  local  battery  circuit  and  its  secondary  terminals  con- 
nected to  the  grid  and  filament  of  a  second  valve  by  which  the  incoming  radio  signal 
is  amplified  by  repetition  of  the  action  shown  in  Fig.  180. 


Fig.     181 — "Repeater"     or     "Regenerative"     Vacuum 
Valve   Circuit. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


161 


The  three  element  vacuum  valve  connected  in  a  simple  tuning  circuit  for  the  reception 
of  spark  signals  is  adjusted  for  the  maximum  strength  of  signals  as  follows:  (1)  The 
incandescence  of  the  lamp  filament  is  carefully  adjusted  by  the  filament  rheostat;  (2)  The 
voltage  of  the  local  battery  is  varied  until  the  maximum  strength  of  signals  is  obtained. 
To  protect  the  filament  and  assist  adjustment,  modern  sets  have  a  small  ammeter  connected 
in  series  with  the  lamp  filament  by  which  the  degree  of  incandescence  is  definitely  fixed. 

143.  A  Repeater  Vacuum  Valve  Circuit. — A  method  for  repeating  the 
current  of  radio  frequency  flowing  in  the  telephone  circuit  back  to  the  grid  circuit 
of  the  same  bulb  appears  in  Fig.  181.  This  system  has  been  found  to  be  particularly 
successful  in  operation  and  gives  increased  strength  of  signals  from  a  given  sta- 
tion. Part  of  the  energy  of  the  oscillations  of  radio-frequency  flowing  in  the 
telephone  circuit  is  repeated  back  to  the  secondary  circuit  through  the  coupling 
coils  (of  radio- frequency)  L-l  and  L-2,  and  the  amplitude  of  the  grid  oscillations 
thereby  increased.  The  amplified  grid  oscillations  then  react  upon  the  telephone 
circuit,  producing  still  greater  variation  of  the  local  current,  thus  further  reinforc- 
ing the  oscillations  of  the  system. 


Fig.    182 — Audio-Frequency   Repeater   Circuit.        Fig.    183 — Tuned  Regenerative  Vacuum   Valve  Circuit. 


The  complete  process  does  not  interfere  with  the  action  of  the  valve  as  a  detector, 
which  at  the  same  time  charges  the  grid  condenser  in  the  usual  fashion,  but  with  in- 
creased strength,  due  to  the  repeating  action  of  the  system. 

To  permit  the  oscillations  of  radio-frequency  to  flow  across  the  telephones,  the  con- 
denser C-3  is  connected  in  shunt.  This  circuit  will  function  without  the  condenser  C-3, 
the  necessary  capacity  being  found  in  the  parallel  cords  of  the  telephone. 

The  current  of  audio-frequency  flowing  in  the  local  telephone  circuit  may  be  repeated 
to  the  grid  circuit  by  the  method  shown  in  the  diagram  of  connections,  Fig.  182,  where 
a  coil  of  audio-frequency  (one  of  large  inductance  values)  L-l  is  coupled  inductively 
to  a  similar  coil,  L-2,  which  is  connected  in  series  with  the  secondary  winding  of  the 
receiving  tuner.  C-2  permits  the  oscillations  of  radio-frequency  to  pass  to  the  grid  and 
filament  and  also  serves  to  tune  the  circuit  L-2,  C-4  to  the  audio-frequent  current  of  the 
local  circuit.  Although  in  this  case  an  audio-frequent  current  is  also  dealt  with,  the  action 
is  identically  the  same  as  in  repeating  back  currents  of  radio-frequency. 

A  circuit  has  been  evolved  by  Armstrong  where  both  the  radio-  and  audio- 
frequent  currents  can  be  simultaneously  repeated  back  to  the  grid  circuit  and 
although  considerable  amplification  of  the  incoming  signals  is  thus  obtained,  the 
system  has  been  found  difficult  to  keep  in  stable  operation. 

Referring  again  to  the  circuit  diagram,  Fig.  181  (where  the  current  of  radio-frequency 
in  the  local  telephone  circuit  is  repeated  back  to  the  grid  circuit) ,  an  improvement  can 
be  effected  by  the  connections  shown  in  the  diagram,  Fig.  183,  where  the  local  circuit 
is  tuned  to  resonance  with  the  grid  or  secondary  circuit  by  the  tuning  coil  L-4.  The 
condenser  C-3  shunts  the  head  telephone  and  battery,  while  the  coil  L-l  repeats  the 
oscillations  of  radio-frequency  back  to  the  grid  circuit  through  the  coil  L-2.  The  circuit 
is  suitable  for  the  reception  of  damped  or  undamped  oscillations,  and  has  been  found  to 
magnify  the  incoming  signal  several  hundred  times. 


162 


PRACTICAL   WIRELESS   TELEGRAPHY. 


It  will  be  noted  from  the  foregoing  diagrams,  Figs.  179  to  183,  and  the  explanations  given, 
that  three  principal  circuits,  which  may  be  termed  the  primary,  secondary  and  tertiary  cir- 
cuits, must  be  placed  in  resonance,  the  tertiary  circuit  being  the  local  telephone  circuit,  with 

its  tuning  coils  and  condensers.  The 
three  circuits  are  not  only  adjusted  (in 
the  case  of  the  reception  of  spark  signals) 
to  substantial  resonance,  but  in  addition 
the  coupling  of  the  repeating  transformer 
must  be  carefully  adjusted,  as  well  as  the 
coupling  between  the  primary  and  secon- 
dary windings.  Correct  values  of  induc- 
tance and  capacity  must  be  selected  in  each 
of  the  three  circuits  for  any  particular 
wave  length.  Beyond  the  tuning  of  the 
third  circuit,  the  general  mode  of  opera- 
tion is  practically  the  same  as  with  the 

ordinary   radio-frequency   tuners, 
rig.     184 — Simple    Regenerative    Circuit.  T       ,        ,,  .  ,  .          , 

It  should  not  require  explanation  that 

with  the  amplification  of  signals  obtained  by  the  circuits  shown  in  Figs.  179  to  183  the  useful 
range  of  transmitting  apparatus  has  been  considerably  increased. 

Another  circuit  for  the  amplification  of  incoming  radio  signals  is  shown  in  the  diagram, 
Fig.  184,  wherein  the  principal  point  to  be  observed  is  the  connection  of  the  secondary 
terminals  of  the  receiving  tuner  to  the  vacuum  valve. 

One  terminal  is  connected  through  the  grid  condenser  to  the  grid  and  the  opposite  ter- 


VALVE  1 


VALVE  2 


VALVE  5 


Li  L-Z  Ls          L4 

Fig.  185 — Vacuum  Valves  Connected  in  Cascade. 


minal  to  the  plate  of  the  valve  instead  of  to  the  filament.  The  local  battery  B-2  and  the 
head  telephone  P,  are  shunted  by  a  condenser  C-3,  and  an  additional  condenser  C-4  is 
connected  between  the  grid  and  filament  of  the  valve.  The  mode  of  operation  of  this  circuit 
probably  does  not  differ  from  the  repeating  circuits  previously  described,  because,  as  will 
be  seen,  there  is  a  certain  amount  of  electrostatic  and  magnetic  coupling  between  the  second- 
ary and  tertiary  (local  telephone  circuit) 
circuits.  The  system  is  adjusted  in  the  fol- 
lowing manner :  The  secondary  circuit,  in- 
cluding the  secondary  condenser,  is  tuned 
to  resonance  with  the  primary  circuit  and 
the  incoming  wave.  The  condenser  C-l  is 
set  at  very  low  values  of  capacity,  and 
the  capacity  of  C-3  varied,  also  C-4,  until 
maximum  response  in  the  head  telephone 
circuit  is  obtained.  Like  ordinary  vacuum 
valve  circuits,  the  potential  of  the 
battery  B-2  must  be  carefully  regulated 
and  the  valve  worked  below  the  charac- 
teristic "blue  glow"  point.  The  vacuum 


•  PHONES  i 


Fig.   186 — Cascade  Connection  of  Vacuum  Valve  with 
One    Lighting    Battery. 

valve  connected  in  this  manner  is  suitable  for  the  reception  of  either  damped  or  undamped 
oscillations. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


163 


144.  The  Vacuum  Valve  Amplifier.- — The  incoming  signals  made  audible 
by  the  simple  vacuum  valve  circuit  shown  in  Fig.  .179,  may  be  amplified  consider- 
ably by  connecting  several  oscillation  valves  in  cascade  as  per  the  diagram,  Fig. 
185.  The  fluctuations  of  current  in  the  local  circuit  are  impressed  upon  the  grids 
of  a  second  and  third  valve  where  due  to  the  relaying  or  trigger  action  of  the  bulb, 
progressive  amplification  is  obtained. 


Fig.    187 — Circuit    for    Cascade   Amplification    of    Radio    Freqtu 


In  the  diagram,  Fig.  185,  in  which  the  left-hand  part  of  the  circuit  starts  with  the  sec- 
ondary winding  of  the  receiving  transformer,  the  telephones  are  removed  from  the  local 
battery  circuit  and  replaced  by  an  iron  core  inductively  coupled  transformer  of  audio-fre- 
quency dimensions.  The  primary  winding  L-l  is  connected  in  series  with  the  battery  B-2, 
but  the  terminals  jpf  the  secondary  winding  L-2  are  connected  to  the  grid  and  filament  of 
the  second  vacuum  valve.  The  circuit  continues  in  this  manner  throughout  several  bulbs, 
but  generally  the  fluctuations  of  current  in  the  third  or  fourth  step  are  strong  enough  to 
paralyze  the  valve,  particularly  if  the  vacuum  is  not  sufficiently  good.  Usually  this 
system  does  not  function  well  above  the  third  step.  To  prevent  the  accumulations  of  extra 
high  potentials  on  the  grids  of  the  amplifying  valves,  an  extremely  high  resistance,  R  (of 
from  500,000  to  1,500,000  ohms  resistance)  may  be  connected  from  the  grid  to  the  filament. 
It  is,  of  course,  understood  that  the  incandescence  of  the  filament  of  each  vacuum  valve  will 
be  carefully  regulated  by  a  small  rheostat  and  the  voltage  of  the  local  battery  regulated  until 
the  best  operating  condition  for  the  individual  bulb  is  obtained. 

If  the  diagram  of  connections,  Fig.  186,  are  adopted,  a  single  storage  battery  may  be 
employed  to  light  all  valve  filaments.  It  will  be  noted  that  the  secondary  circuit  of  the 
audio-frequency  transformer  L-2,  L-3  is  conductively  open,  and  although  it  is  somewhat' 
difficult  to  explain  exactly  how  this  transformer  functions,  good  results  are  reported  to  be 
obtained  by  means  of  it.  Either  of  the  foregoing  types  of  amplifiers  may  be  used  to  step 
up  the  signals  of  an  ordinary  wire  telephone  as  well  as  the  signals  of  wireless  telegraphy. 

145.  Amplification  of  Radio  Frequencies. — By  means  of  a  cascade  connec- 
tion of  vacuum  valves,  we  may  amplify  the  oscillations  of  radio  frequency,  trans- 
ferring them  from  one  bulb  to  another  until  the  desired  increase  of  signal  is  ob- 
tained. A  satisfactory  diagram  of  connections  appears  in  Fig.  187. 

The  circuits  of  radio-frequency  are  similar  to  that  of  any  receiver,  with  the  exception 
that  a  small  value  of  potential  from  a  local  battery  is  applied  to  the  filament  and  grid  of 
each  vacuum  valve,  the  E.  M.  F.  being  adjusted  by  the  potentiometers  P-l  and  P-2.  The  in- 
coming oscillations  amplified  by  the  first  vacuum  valve  are  transferred  into  the  second 
vacuum  valve  through  the  radio-frequency  transformer  L-5,  L-6.  The  circuit  of  L-5  is  tuned 
to  resonance  with  the  incoming  oscillations  by  the  condenser  C-5  and  the  secondary  circuit 
L-6,  C-6  tuned  to  the  same  frequency  by  means  of  C-6.  By  proper  adjustment  of  the  volt- 
ampere  characteristic  of  the  second  bulb  (by  the  potentiometer  P-2)  the  effect  of  the  in- 
coming oscillations  is  to  place  a  charge  in  the  condenser  C-7  in  shunt  to  the  head  telephones, 


164 


PRACTICAL   WIRELESS   TELEGRAPHY. 


which  in  turn  discharges  through  the  windings  of  the  telephones,  causing  a  single  sound.f 
More  in  detail,  by  proper  adjustment  of  the  potentiometer  in  the  grid  circuit  of  the  last 
valve,  the  amplitude  of  the  positive  half  of  the  radio-frequent  current  repeated  in  the 
tertiary  or  local  telephone  circuit  exceeds  the  amplitude  of  the  negative  half,  and,  due  to 
this  fact,  more  current  flows  in  one  direction  than  in  the  other;  accordingly,  the  telephone 
condenser  C-7  becomes  charged  over  a  period  corresponding  approximately  to  that  of  the 
duration  of  a  wave  train  and  then  probably  discharges  in  one  direction  through  the  head 
telephones  at  rates  corresponding  to  the  spark  frequency  of  the  transmitter.* 

146.  The  Effects  of  Distributed  Capacity. — Every  coil  of  wire  possesses  a 
certain  amount  of  distributed  capacity  (sometimes  termed  self -capacity),  e.  g., 
energy  is  stored  up  between  adjacent  turns  during  the  flow  of  oscillations  in  the 
form  of  electrostatic  lines  of  force,  which  produce  an  effect  similar  to  connecting 
a  small  condenser  in  shunt  to  the  coil. 

Thus  a  tuning  coil  of  given  dimensions,  having  considerable  self -capacity,  will  have  a 
_ — __  1.  i  -  .  defined  time  period  of  oscillation  and  will 


CD 
CD 


Fig.  188 — Showing  the 
"Dead-end"  or  "End" 
Turns  of  a  Receiver  Tun- 
ing Coil. 


Fig.  189 — Showing  the 
Position  of  an  End  Turn 
Switch. 


respond  more  freely  to  one  impressed 
frequency  than  to  another.  Now,  it  is 
found  by  experiment  that  a  crystal  de- 
tector, such  as  the  carborundum  rectifier, 
gives  the  loudest  response  in  the  receiv- 
ing telephone  when  the  maximum  possi- 
ble voltage  for  a  given  group  of  incoming 
oscillations  is  impressed  upon  it,  and  gen- 
erally a  secondary  receiver  circuit  in 
which  inductance  predominates  gives  the 
maximum  possible  voltage.  Then  if  a 
secondary  circuit  minus  a  shunt  condenser 
is  constructed  so  that  its  natural  oscilla- 
tion frequency  accords  with  the  frequency 
of  the  oscillations  flowing  in  the  trans- 
mitter aerial,  maximum  response  is  se- 
cured. All  this  may  be  summed  up  by 
saying  that  a  secondary  receiver  circuit 
similar  to  one  in  which  it  is  supplied,  the 
A  primary  winding  having 


lacking  a  shunt  variable  condenser  functions 
necessary  capacity  being  found  in  the  self-capacity  of  the  coil, 
distributed  capacity  is  somewhat  objectionable  for  use  in  tuning  circuits,  because  an  appre- 
ciable amount  of  current  will  oscillate  through  the  unused  turns,  particularly  if  the  fre- 
quency of  the  incoming  signal  equals  the  natural  frequency  of  the  coil.  Coils  made  of  fine 
wire  or  those  wound  irregularly  tend  to  reduce  the  effects  of  distributed  capacity. 

147.     The  End  Turns  of  a  Receiving  Tuner  and  End  Turn  Switches. — 

In  the  preceding  paragraph  the  effect  of  distributed  capacity  in  the  windings  of  a  tuning  coil 
was  discussed,  and  the  fact  mentioned  that  a  coil  of  given  dimensions  may  have  a  well-defined 
time  period  of  oscillation  and  may  therefore  oscillate  more  freely  to  an  impressed  current  of 
a  particular  frequency. 

Assume  that  such  a  coil  is  connected  in  series  with  the  antenna  circuit  shown  in  Fig.  188 
and  that  for  resonant  adjustment  to  the  wave  length  of  600  meters  the  turns  between  points 
A  and  B  only  are  required.  The  remaining  turns  from  B  to  C  are  therefore  useless  for 
this  particular  wave  length,  and  should  the  coil  have  a  natural  time  period  of  oscillation 
corresponding  to  the  wave  of  600  meters,  an  appreciable  amount  of  current  would  oscillate 
backward  and  forward  through  the  unused  or  end  turns.  This  in  effect  amounts  to  placing 
a  secondary  circuit  in  resonance  with  the  antenna  circuit  and  results  in  the  absorption  of 
some  of  the  energy  of  the  oscillations. 

If  the  antenna  system  were  only  to  be  tuned  to  the  maximum  wave  length  of  600  meters, 
the  additional  turns  on  the  coil  B  to  C  would  not  be  required,  but  inasmuch  as  the  aerial 
system  may  have  to  be  tuned  to  various  wave  lengths  up  to  3,000  or  4,000  meters,  the  full 
number  of  turns  are  required  for  the  winding. 

If  then,  for  example,  the  coil  A,  C,  is  interrupted  at  the  point  D,  E,  as  shown  in  Fig.  189, 
the  unused  turns  of  the  winding  are  conductively  disconnected  from  the  used  turns  and 

t  Note  also  the  radio-frequency  amplifier  described  in  paragraph  226.      Note  also  diagram  Fig.  296. 
*  Explanation  according  to  Armstrong. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


165 


therefore  a  considerable  amount  of  current  which  heretofore  oscillated  through  the  unused 
turns  is  now  converted  to  a  more  useful  purpose. 

All   this   leads  to  the  point  that  the  maximum   diegree  of  efficiency  and  selectivity   is 
obtained  from  a  receiving  tuner  having  properly  located  "end  turn"  switches  fitted  to  the 

primary  and  secondary  coils.  It  should, 
however,  be  remembered  that  merely  dis- 
connecting the  unused  turns  does  not 
completely  prevent  the  absorption  of 
energy  by  that  part  of  the  circuit,  but  it 
tends  to  reduce  the  absorption  to  a 
minimum. 

An  "end-turn"  switch  may  take  one  of 
several  designs  but  the  fundamental  con- 
struction of  the  type  used  in  the  latest 
tuners  manufactured  by  the  American 
Marconi  Company  is  shown  in  the  dia- 
gram, Fig.  190,  where  the  successive  groups 
of  a  primary  or  secondary  tuning  coil  are 
represented  at  1,  2,  3  and  4.  Mounted 
on  a  hard  rubber  drum  are  the  copper 
segments  S-2,  S-3  and  S-4,  which  are 
slightly  offset  so  that  the  successive 
units  of  the  tuner  winding  are  connected 
in  the  circuit  when  the  knob  is  turned 
counter  clockwise.  The  circuit  is  com- 
pleted by  brushes  A  and  B  which  make 
TAPS  FOR  STUD5  UNDER  BLADE  5  .  contact  with  segments  S-2  and  brushes  C 

Fig.  190— Sketch  Showing  the  Construction  of  "End       and  D  which  make  contact  with  S-3   and) 

so  on.      In  addition  the  switch  blade   S-l 

makes  contact  with  the  taps  of  a  multipoint  switch  (n.ot  shown)  which  cuts  in  groups,  10 
turns  at  a  time.  For  example,  when  the  switch  S-l  has  been  turned,  let  us  say,  counter 
clockwise  until  all  the  turns  of  section  No.  1  are  cut  into  the  circuit  then  segment  S-4  makes 
contact  with  brushes  E  and  F,  connecting  section  No.  2  in  the  circuit.  The  blade  of  the 
multipoint  switch  S-l  then  progresses  across  the  contacts  connected  to  the  turns  of  section  2 
and  when  the  last  stud  has  been  passed,  the  circuit  is  completed  through  the  next  group  by 
brushes  C,  D,  which  make  contact  with  segments  S-3.  The  switch  blade  S-l  then  pro- 
gressively cuts  in  the  additional  turns  of  section  No.  3  and  so  on  until  S-2  cuts  in  group 
No.  4. 

In  the  design  of  a  commercial  re- 
\ceiving  tuner,  the  primary  or  second- 
ary winding  is  interrupted  at  points 
suitable  to  the  standard  wave  lengths. 
For  example,  if  sections  1,  2,  3  and  4 
are  the  secondary  units  of  a  receiving 
tuner,  the  winding  is  interrupted  be- 
tween 1  and  2  at  such  a  point  that  the 
wave  length  of  the  closed  circuit 
(with  small  values  of  capacity  in 
shunt)  will  be  slightly  above  600 
meters.  The  next  interruption  of  the 
circuit  will  be  made  at  a  point  where 
the  wave  length  adjustment  is  slightly 
above  1,000  meters,  the  next  for  2,500 


Fig.   191 — Showing  How  the  Inductance  of  the  Radio 

Frequency    Tuning    Coil    Can    Be    Varied    by 

One  Turn   at   a   Time. 


meters,  and  so  on.  The  foregoing  selections  are  those  agreeable  to  American 
practice  where  ships  communicate  with  naval  or  Marconi  ship  and  shore  stations. 
148.  The  Variation  of  a  Radio  Frequency  Inductance. — In  the  earlier 
types  of  receiving  tuners,  the  inductance  of  the  radio-frequency  coils  was  varied 
by  mounting  a  sliding  contact  on  a  brass  rod,  the  contact  passing  over  successive 
turns  of  bare  wire.  It  is  obvious  that  such  a  design  is  mechanically  incorrect  be- 


166 


PRACTICAL   WIRELESS   TELEGRAPHY. 


O 


cause  the  gliding  contact  soon  cuts  through  the  wire  either  actually  breaking  the 
turns  or  short  circuiting  adjacent  turns. 

By  the  method  shown  in  the  diagram,  Fig.  191,  the  inductance  of  a  single  layered  coil 
can  be  progressively  varied  from  one  turn  to  the  maximum  number  simply  through  the 
use  of  two  multipoint  switches  doing  away  with  the  sliding  contact. 

For  purposes  of  illustration,  assume  a  winding  having  110  turns.  Then  the  first  10  single 
turns  are  connected  to  the  contact  points  of  the  switch  S-2  and  the  remaining  turns  are 
connected  in  groups  of  10  to  the  contact  points  of  the  switch  S-l.  An  odd  number  of  turns 

can  be  selected  in  the  following  manner : 
If  87  turns  are  required  for  resonance  in 
any  particular  circuit,  switch  S-l  is  set 
at  the  eighth  contact  point  and  switch 
S-2  on  contact  point  80,  making  87  com- 
plete turns.  One  turn  of  inductance,  for 
instance,  can  be  selected  by  setting  S-l 
at  zero  and  S-2  at  contact  point  1.  This 
is  the  method  by  which  the  inductance 
of  the  primary  winding  of  the  Marconi 
type  106  receiving  is  varied. 

It  is  not  usual  to  vary  the  inductance 
of  the  secondary  circuit  of  a  receiving 
tuner  in  this  manner.  The  method  gen- 
erally employed  is  to  vary  the  inductance 
by  groups  of  10  or  15  turns  at  a  time, 
the  necessary  closeness  of  adjustment 
between  groups  being  obtained  by  the  shunt 
variable  condenser  for  sharp  tuning.^ 

Extremely  close  variation  of  the 
inductance  of  a  radio-frequency  cir- 
cuit can  be  obtained  by  the  instru- 

rig.    192 — 1  he    Construction   of   the    Variometer.  ,        ,  •'.  ,, 

ment    termed    the    variometer,    the 
principle  of  which  has  already  been  explained  in  connection  with  the  transmitter. 

A  variometer*  suitable  for  receiving  apparatus  is  shown  in  Figs.  192  and  193,  where  a 
stationary  coil  A  has  mounted  within  it  a  ball  winding  B  which  rotates  on  a  metal  shaft. 
A  knob  and  pointer  is  attached  to  the  shaft 
and  a  suitable  scale  supplied.  The  ball 
windings  A  and  B  are  connected  in  series 
and  if,  in  one  concentric  position,  they  are 
connected  so  that  their  magnetic  fields  op- 
pose the  inductance  of  the  variometer  will 
be  practically  zero,  but  if  the  inner  ball  is 
turned  on  its  axis,  the  inductance  will 
gradually  increase,  maximum  inductance 
being  obtained,  when  the  inner  coil  has 
completely  changed  sides  with  respect  to 
the  outer  coil.  For  receiving  apparatus,  the 
inner  and  outer  balls  are  generally  wound 
with  No.  24  B.  and  S.  wire  if  connected  in 
the  antenna  circuit  and  with  No.  32  B.  and 
S.  wire,  if  connected  in  series  with  the 
secondary  or  detector  circuit. 

Although  the  variometer  is  a  very 
useful  instrument  for  tuning  the  re- 
ceiver, it  possesses  the  disadvantage 
of  offering  considerable  resistance  to 
the  passage  of  radio-frequent  cur- 
rents when  used  at  inductance  values 
near  to  zero.  That  is  to  say,  a  sim- 


'  BINDING  POST 


POINTER 


SCME 


'Fig.   193 — The  Tuning  Variometer  Complete. 


*See    paragraph    30;    also    fig.    18. 
fSee    section    H    appendix. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


167 


Fig.  194 — Buzzer  Excitation  System. 


pie  tuning  coil  of  the  sarrie  value  of  inductance,  would  obviously  possess  less 
resistance. 

149.  Buzzer  Excitation  Systems. — If  any  part  of  an  active  vibrating  buzzer 
(energized  by  a  battery)  is  placed  in  inductive  relation  to  either  the  open  or  closed 
circuits  of  a  receiving  set,  the  fluctuations  of  the  battery  current  act  inductively 
on  the  receiving  circuits  and  set  up  therein  feeble  currents  which  are  rectified  and 
made  audible  in  the  head  telephone. 

Maximum  response  in  the  head  telephone  is  obtained  when  the  crystal  rectifier 
is  adjusted  for  the  best  degree  of  rectification,  hence  the  buzzer  affords  a  ready 

means  for  readjustment  of  a  receiv- 
ing detector  of  any  type.  A  perfect 
reproduction  of  the  tone  of  the  buzzer 
is  obtained  in  the  head  telephones,  the 
pitch  varying  in  accordance  with  the 
interruptions  of  the  vibrator. 

In  one  method  a  wire  is  extended 
from  some  part  of  the  buzzer  circuit, 
preferably  the  contact  points,  and  is  con- 
nected directly  to  either  the  earth  lead  of 
the  receiving  tuner  or  to  one  terminal  of 
the  secondary  circuit,  but  the  better 
method  is  to  place  some  part  of  the  cir- 
cuit leading  to  the  buzzer  in  inductive 
relation  to  the  antenna  system  as  in  the 
diagram,  Fig.  194.  In  this  diagram  a 
small  coil  L-2  of  6  or  7  turns  of  wire 
(about  lJ/2  inches  in  diameter)  connected 
in  series  with  the  antenna  system  has 
wound  around  it,  7  turns  comprising  the 
coil  L-l.  Coil  L-l  is  in  series  with  the  battery  and  the  buzzer,  and  consequently  when  the 
buzzer  is  set  into  action,  a  change  of  flux  takes  place  through  L-l  which  sets  up  a  difference 
of  potential  across  L-2  and,  therefore,  charges  the  antenna  system,  causing  it  to  oscillate  at 
the  particular  frequency  to  which  it  is  adjusted.  Then,  when  the  secondary  circuit  of  the 
receiving  tuner  is  adjusted  to  resonance  with  the  antenna  circuit  and  the  crystal  detector 
adjusted  for  the  most  sensitive  operating  condition,  the  louder  sound  will  be  obtained  in 
the  head  telephone. 

With  supersensitive  receiving  apparatus  it  is  sometimes   necessary  to  place  the  buzzer 
excitation  circuit  several  feet  from  the  windings  of  the  receiving  tuner  to  prevent  the  sound 
in   the    head   telephone    from   becoming   of   un- 
bearable intensity. 

Although  various  means  have  been  devised 
to  shield  crystalline  detectors  from  the  local 
transmitting  apparatus  at  a  given  station,  none 
have  as  yet  afforded  the  desired  protection. 
Hence,  it  is  generally  necessary  at  the  close  of 
a  period  of  transmission  to  readjust  the  re- 
ceiving detector  to  -its  maximum  degree  of 
sensitiveness.  If  the  receiving  tuner  is  fitted 
with  a  test  buzzer  the  detector  can  be  quickly 
readjusted  by  setting  the  buzzer  into  action  and 
changing  the  position  of  the  opposing  contact 
to  the  crystal  until  the  loudest  signals  are  ob- 
tained. 

The  ordinary  vibrating  buzzer  can  be 
put  to  a  variety  of  uses  in  radio  telegraph 
systems,  such  as  exciting  the  circuits  of  a 
wavemeter  or  setting  up  oscillations  in  an  aerial  system  for  radio  telegraph  meas- 
urements.   Such  circuits  will  be  described  in  the  chapter  following. 

150.  Receiving  Telephones. — The  flux  generated  by  a  given  electromagnet 


Fig. 


194a— 3,000-Ohm     Head     Telephone 
Receiver. 


168 


PRACTICAL   WIRELESS   TELEGRAPHY. 


-B- 


is  determined  by  the  product  obtained  in  multiplying  the  strength  of  the  current 
by  the  number  of  turns,  e.  g.,  the  ampere  turns  determine  the  magnetizing  flux. 
Hence,  if  a  feeble  current  only  is  available,  a  magnet  wound  with  a  large  number 
of  turns  of  fine  wire  will  generate  the  greatest  magnetizing  flux. 

Now  the  local  circuit  of  the  average  telephone  receiver,  such  as  shown  in  Fig.  194a, 
is  traversed  by  feeble  currents,  and  therefore  the  magnets  of  the  telephones  should 
be  wound  with  a  great  number  of  turns  of  rather  fine  wire  such  as  No.  36  or  No.  40 
B.  and  S.  Such  a  winding  necessarily  has  considerable  resistance,  but  a  greater  magnetizing 

force  will  result  therefrom  than  would  be  ob- 
tained by  a  winding  of  a  few  turns  of  coarser 
wire.  The  resistance  of  the  average  telephone 
receiver  for  reception  of  radio  signals  is  2,000 
to  3,000  ohms.  These  receivers  are  generally 
more  sensitive  than  the  type  employed  in  land 
line  telephony;  but  it  should  be  borne  in  mind 
that  it  is  not  alone  the  magnetizing  flux  gen- 
erated that  determines  the  volume  of  sound 
produced  by  the  telephone,  but  the  overall  con- 
struction influences  its  sensitiveness  as  well. 
This  includes  particularly  the  weight  of  the 
diaphragm  which  in  radio  receivers  is  generally 
lighter  than  the  diaphragm  used  in  the  ordinary 
wire  telephone  receiver. 

A  series  of  experiments  conducted  by 
Dr.  Austin  indicate  that  the  average  tele- 
phone produces  the  greatest  volume  of 
sound  when  the  frequency  of  the  alternat- 
ing current  passed  through  the  windings 
varies  from  300  to  500  cycles  per  second. 
Frequencies  in  excess  of  500  cycles  de- 
crease the  amplitude  of  vibration  of  the 
telephone  diaphragm  and  hence  produce 
less  sound. 

In  view  of  the  foregoing,  a  transmitter 
having  a  spark  frequency  of  600  to  1,000 
sparks  per  second  gives  better  response  in 
the  head  telephones  than  one  of  lower 
frequency.  Part  of  this  is  due  to  the  fact 
that  the  human  ear  is  more  responsive  to 
the  higher  frequencies  than  to  those  of 
lower  pitch. 

Telephones  of  75  to  150  ohms  resist- 
ance, if  used  with  the  magnetic  detector, 
the  tikker,  and  the  three  element  vacuum 
valve  gives  good  response,  but  other  re- 
ceiving detectors,  such  as  the  crystal  recti- 
fiers, require  telephones  of  at  least  2,000) 
ohms  resistance.  Telephones  of  15,000) 
ohms  resistance  have  been  constructed  but: 
such  a  winding  is  not  required  for  the 
average  radio  receiver. 

A  telephone  receiver  possessing  a  notable 
degree  of  sensitiveness  has  been  recently  developed  in  the  United  States  by  T.  Baldwin. 
The  essential  parts  of  this  receiver  are  indicated  in  the  diagram,  Fig.  195a,  where  a  ring: 
shaped  horseshoe  magnet  with .  poles  N  and  S  is  fitted  with  two  "U"  shaped  soft  iron 
pieces,  P-l  and  P-2.  The  telephone  winding  W,  placed  longitudinally  between  these  pole 
pieces,  has  a  slot  in  the  center  in  which  is  placed  a  soft  iron  armature  balanced  on  the 
pivot  P-3.  One  end  of  the  soft  iron  armature  is  connected  by  the  brass  wire  R  to  the  mica 


-c- 


-D- 


Fig.    195a,    b,    c    Construction    and    Circuit 
the    Baldwin   Telephone. 


kECElVlNG  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


169 


Or  isinglass  diaphragm  M  which  is  screwed  under  the  receiver  cap  as  in  the  ordinary 
telephone.  During  the  passage  of  a  fluctuating  current  through  the  winding  W  the  soft 
iron  armature  A  is  thrown  into  vibratory  motion  and  the  vibrations  transferred  to  M. 
A  prominent  feature  of  this  receiver  is  the  fact  that  the  armature  A  is  under  no  magnetic 
strain  until  a  fluctuating  current  flows  through  the  ^vinding  W. 

The  fundamental  principle  of  operation  is  as  follows :  The  diagram,  Fig.  195b,  shows 
the  direction  of  the  lines  of  force  of  the  permanent  magnet  through  the  pole  pieces,  and 
as  will  be  observed  the  flux  divides  equally  between  both  sides  of  the  "U"  shaped  poles 
and  continues  through  the  magnet.  Due  to  this  division  of  flux,  there  is  no  strain  on  the 
armature  A,  a  condition  just  opposite  to  that  found  in  the  ordinary  telephone  receiver, 
where  the  diaphragm  is  drawn  toward  the  core  of  the  magnet  and  held  under  considerable 
tension. 

If  current  flows  through  the  winding  in  a  given  direction  and  the  permanent  magnet 
were  removed,  the  resultant  flux  would  have  the  direction  indicated  by  the  arrows,  Fig. 
195c,  that  is,  the  direction  of  the  flux  would  be  lengthwise  of  the  winding  and  would  cir- 
culate in  the  direction  shown.  Under  such  conditions,  the  armature  would  be  equally 
attracted  to  the  lower  and  upper  pole  pieces  and  remain  in  a  neutral  position. 

If  a  direct  current,  let  us  say,  circulates  through  the  winding  from  an  external  source, 
as  in  Fig.  195d,  then  the  lines  of  force  issuing  from  the  permanent  magnet  and  from  the 
electromagnet  will  oppose  in  some  parts  of  the  magnetic  circuit,  but  in  other  parts  of  the 
circuit  they  will  flow  in  the  same  general  direction.  In  the  particular  example  cited,  the 
fluxes  will  oppose  on  the  left  hand  side  of  the  upper  pole  piece  and  in  consequence  the  lines 
of  force  will  flow  to  the  right  hand  pole  piece  and  from  that  point  on  circulate  through 

the  core  of  the  perma- 
nent  magnet,  as  shown 
by  the  long  arrows. 

The  effect  of  this 
will  be  to  attract  the 
armature  A  to  the  left 
hand  side  of  the  lower 
pole  piece  and  to  the 
right  hand  side  of  the 
upper  pole  piece  and  the 
mica  diaphragm  will 
then  be  deflected  up- 
ward. .  If  current  is 
passed  through  the 
winding  in  the  opposite 
direction,  the  actions  just 

Fig.     196— Diagram    Showing    the    Principle    of    Microphonic    Relay.  described       Will      be      re- 

versed  and  the  dia- 
phragm will  be  deflected  in  the  opposite  direction.  The  entire  action  is  thus  seen  to  be 
much  similar  to  a  polarized  relay. 

The  sensitivity  of  this  telephone  can  be  accounted  for  first,  by  the  fact  that  the  magnetic 
circuit  is  one  of  low  reluctance;  second,  the  armature  of  the  magnet ^is  under  no  strain 
until  current  passes  through  the  receiver  winding,  hence  a  greater  deflection  of  the  dia- 
phragm is  obtained;  third,  the  armature  is  acted  upon  at  both  ends  and  since  the  flux  is 
produced  differentially  like  that  in  a  polarized  relay,  the  deflection  for  a  given  magnetizing 
current  is  accordingly  increased. 

151.  Microphonic  Relays  or  Sound  Intensifies. — A  microphonic  relay  has 
been  developed  for  intensifying  the  currents  flowing  in  the  local  circuit  of  a  wire- 
less telegraph  receiver,  but  owing  to  the  difficulty  experienced  in  maintaining  the 
instrument  in  a  permanent  state  of  adjustment,  it  has  not  been  commercially 
adopted. 

A  working  idea  of  the  relay  may  be  obtained  from  diagram,  Fig.  196,  wherein  the  tele- 
phones are  removed  from  the  local  circuit  of  a  given  receiving  set  and  the  microphonic 
relay  winding  W  substituted.  Immediately  in  front  of  the  magnet  is  placed  a  diaphragm 
which  may  be  tuned  mechanically  to  the  spark  frequency  of  the  transmitter. 

The  diaphragm  carries  the  carbon  button  A;  the  second  carbon  button  B  is  attached 
to  an  adjustable  screw,  the  two  buttons  being  placed  in  light  contact  with  each 
other.  A  local  battery  of  1^2  to  6  volts  is  joined  in  series  with  the  microphonic  contact 


170 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Fig.   197a — Brown  Amplification  Relay  for  Increasing 
Radio   Signals. 


and  the  circuit  continues  through  the  head  telephones  P-l.  During  the  reception  of  radio 
signals  the  vibration  of  the  diaphragm  P  causes  greater  vibration  of  the  diaphragm  of  the 
head  telephone  P-2  due  to  the  larger  value  of  current  flowing  through  the  local  circuit  than 
through  the  circuit  of  B-l. 

If  a  number  of  these  relays  are  connected  in  cascade,  the  signals  will  be  increased  from 
20  to  30  times  their  original  strength.  Extremely  careful  adjustment  of  the  microphonic 
contacts  is  required  for  the  best  response. 

152.  Brown   Amplifying    Relay. — A    particularly    successful    microphonic 

relay  has  been  developed  by  S.  G. 
Brown.  This  relay  has  been  employed 
by  the  Marconi  Company  for  trans- 
oceanic communication  in  connection 
with  carborundum  crystals  with  good 
results. 

The  approximate  mechanical  construc- 
tion and  the  electrical  circuit  of  one  type  is 
shown  in  Fig.  197a,  where  the  soft  iron 
cores  K,  K,  are  magnetized  by  a  perma- 
nent magnet  with  the  poles  N  and  S.  A 
fine  wire  winding  W-l  encircles  the  upper 
end  of  the  core  and  a  larger  magnet  wind- 
ing W-2  the  lower  end.  Directly  above  the 
core  of  W-l  is  a  steel  vibrating  tongue  R 
carrying  the  contact  D,  which  touches 
lightly  contact  C,  the  latter  being  adjust- 
able by  a  screw.  The  contact  mounted  on 
the  screw  adjustment  is  made  of  a  special  osmium-iridium  alloy  and  the  other  contact  is 
a  button  of  carbon  which,  when  pressed  lightly  against  the  upper  point  constitutes  a 
microphone.  Winding  W-2,  telephone  P,  dry  cell  battery  B,  are  all  connected  in  series 
with  the  microphonic  contact.  The  current  to  be  amplified  enters  winding  W-l  at  A  ami  C 
and  the  resulting  change  of  flux  in  the  core  of  the  magnet  moves  the  vibrating  tongue  R, 
which,  in  turn,  varies  the  current  of  B, 
causing  sounds  in  the  head  telephone. 
The  amplitude  of  the  vibration  of  R  is 
increased  by  the  current  of  battery  B  cir- 
culating through  the  winding  W-2.  In 
other  words,  the  fluctuating  microphone 
current  is  repeated  back  to  the  magnet 
through  winding  W-2  and  the  vibrations 
of  R  magnified  accordingly. 

If  three  or  four  of  these  relays  be 
connected  in  cascade  considerable  ampli- 
fication of  the  incoming  signal  will  be  ob- 
tained. For  the  cascade  connection,  the 
telephones  are  removed  from  the  local  cir- 
cuit of  the  first  relay  and  another  winding 
of  the  second  relay  like  that  of  W-l  con- 
nected in  its  place,  and  so  on  throughout 
the  series. 

A  small  milliameter  M  is  connected  in 
series  with  the  telephone  and  when  the 
current  flow  is  8  to  12  milliamperes,  the 
relay  is  in  the  most  sensitive  adjustment. 

A  Brown  relay  of  another  type  is  shown  in  Fig.  197b,  where  M  is  a  microphone  chamber 
filled  with  carbon  granules,  the  pressure  on  which  can  be  carefully  regulated  by  means  of 
an  adjusting  screw.  The  vibrating  tongue  R  is  acted  upon  by  the  flux  of  the  poles  A,  B. 
and  the  corresponding  fluctuations  of  the  battery  B-2  not  only  circulate  through  the  wind- 
ings W-2  and  thereby  amplify  the  vibrations  of  R,  but  these  currents  also  pass  through  a 
step-up  transformer,  having  the  primary  winding  P  and  the  secondary  winding  S.  The 
current  induced  in  S  flows  to  the  condenser  C  (of  about  2  microfarads  capacity)  and  through 
the  head  telephones.  The  battery  B-2  is  one  of  about  6  volts  pressure. 


Fig.   197b — Supersensitive  Brown  Amplification  Relay. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


171 


Like  the  first  type  described,  several  of  these  relays  may  be  connected  in  cascade  for 
progressive  amplification  of  the  incoming  radio  signals. 

153.  Atmospheric  Electricity. — When  the  aerial  wire  of  a  radio  station  is 
suspended  above  the  earth,  it  not  only  absorbs  a  certain  amount  of  energy  from 
passing  electric  waves,  but  it  will  be  set  into  oscillation  by  irregular  discharges  of 
so-called  atmospheric  electricity  which 
is  termed  throughout  the  United  States 
"static"  and  in  foreign  countries  ''at- 
mospherics" or  "strays." 

During  the  spring,  summer  and  autumn 
in  the  northern  zones  of  the  United  States 
the  potential  of  the  air  seems  to  be  higher 
than  that  of  the  earth  and  in  consequence 
a  discharge  takes  place  through  the  aerial 
conductors  to  earth  at  irregular  intervals. 
This  induced  current  causes  a  crackling, 
irregular  sound  of  more  or  less  intensity 
in  the  telephones  of  the  receiving  set  which 
may  interfere  to  a  marked  extent  with  the 
reception  of  radio  signals. 

Widely  varying  theories  exist  as  to  the 
probable  origin  of  static  discharges,  but  a 
considerable  amount  of  this  interference  is 
quite  likely  set  up  by  nearby  or  far  distant 
electric  storms.  Just  as  the  transmitter  of 
a  radio  station  sets  up  electrical  oscilla- 
tions of  more  or  less  damping,  in u  the  re- 


Fig.    197c — Details    Brown'  Relay. 


ceiver  aerial,  so  also  the  discharge  of  lightning  between  two  oppositely  charged  clouds  or 
between  the  clouds  and  the  earth  sets  up  a  highly  damped  electric  wave  which  will  set 
practically  any  receiving  aerial  into  vibration  at  whatever  frequency  it  happens  to  be 
adjusted. 

Static  electricity  is  especially  prevalent  in  tropical  countries  and  in  the  northern  zones 
of  the  United  States  during  the  summer  months  of  the  year,  but  during  the  night  hours 
it  is  apt  to  be  strong  in  the  northern  zones  during  the  colder  months  of  the  year. 

To  reduce  the  interference  of  static  the  transmitting  apparatus  is  designed  to 
produce  a  spark  note  of  high  pitch  having  a  more  or  less  musical  tone.  The  dots 
and  dashes  can  then  be  readily  distinguished  at  the  receiving  station  due  to  the 
difference  in  the  pitches  of  the  interfering  static  discharges  and  that  of  the  spark 
at  the  transmitting  station.  Modern  transmitters  employing  current  of  500 
cycles  produce  a  musical  spark  permitting  communication  to  be  carried  on  under 
conditions  when  it  would  not  be  possible  with  a  spark  note  of  irregular  pitch  such 
as  is  obtained  with  irregularly  acting  spark  gaps. 

When  the  receiving  apparatus  is  connected  to  very  large  aerials,  severe  static 
discharges  are  obtained  day  or  night  throughout  the  year  and  in  order  to  carry 
on  communication  through  this  interference,  a  transmitter  of  very  great  power 
that  will  permit  the  signals  to  be  heard  above  the  static  discharges  is  employed. 

Various  local  appliances  have  been  devised  to  overcome  the  interference  of  static,  but 
none  have  been  so  far  highly  successful.  Balancing  circuits  have  been  devised  by  the 
Marconi  Company  with  which  they  have  achieved  good  results  (described  in  following 
paragraph). 

With  ordinary  receiving  apparatus,  the  operator,  at  any  given  radio  station,  may 
partially  reduce  static  interference  by  employing  the  least  possible  degree  of  coupling  be- 
tween the  primary  and  secondary  windings  of  the  receiving  tuner,  the  respective  windings 
being  mechanically  drawn  apart  to  a  distance  consistent  with  the  strength  of  signals  in  the 
head  telephones.  Occasionally  static  interference  can  be  minimized)  by  throwing  the  primary 
and  secondary  circuits  out  of  resonance  or  in  case  a  carborundum  rectifier  is  employed  as 
the  detector,  by  the  application  of  an  abnormal  local  E.  M.  F. 

With  sensitive  receiving  apparatus,  such  as  the  repeating  three-element  oscilla- 


172 


PRACTICAL  WIRELESS  TELEGRAPHY. 


tion  valve  or  the  step-up  valve  amplifier,  the  static  signals  are  enormously 
amplified  and  the  good  effects  of  these  receivers  are  aften  thus  offset. 
Hence  when  atmospheric  electricity  is  especially  severe,  radio  traffic  may  be  more 
readily  dispatched  from  station  to  station  by  means  of  a  less  sensitive  receiving 
detector,  such  as  the  carborundum  rectifier  or  by  the  Marconi  magnetic  detector. 

An  interesting  aspect  of  the  static 
PI         PZ  interference  is  the  fact  that  it  does 

not  persist  at  certain  months  of  the 
year  in  certain  localities,  being  strong 
for  a  period  of  several  days  and  then 
sometimes  disappearing  for  a  space 
of  a  week  or  more.  This  may  be  fol- 
lowed by  a  period  of  several  days, 
during  which  the  interference  from 
this  source  is  exceptionally  strong, 
but,  as  stated  previously,  the  high 
pitched  note  of  modern  transmitters 
to  a  large  extent  overcomes  the 
difficulty. 

154.  The  Marconi  Balanced  Crystal  Receiver  (English  Marconi  Company). 
—No  apparatus  or  circuit  has  so  far  been  devised  to  completely  eliminate  the  in- 
terference of  static  discharges.  However,  communication  can  be  effected  by  the 
circuits  of  the  Marconi  balanced  crystal  receiver  when  with  ordinary  circuits  it 
would  not  be  possible.  In  fact,  the  balanced  crystal  tuner  often  reduces  the  inter- 
ference from  this  source  to  a  marked  degree. 

The  complete  circuits  of  the  balanced  crystal  receiver  are  shown  in  the  diagram,  Fig. 
198,  where  the  secondary  winding  of  the  receiving  tuner  is  represented  by  the  coil  L-l, 


Fig.    198 — Circuits      of     Marconi's      Balanced      Crystal 
Receiver.      (English    Marconi    Company.) 


the  secondary  condenser 
at  P-l  and  P-2,  the  bat- 
tery at  B  and  the  tele- 
phones at  P. 

To  carry  out  the  fun- 
damental principle  of  this 
receiver,  crystals  D-l 
and  D-2  must  have  like 
volt-ampere  characteris- 
tics, and,  accordingly, 
crystals  are  specially  se- 
lected for  the  purpose. 

Briefly,  the  action  of 
this  circuit  can  be  de- 
scribed as  follows:  If 
crystals  D-l  and  D-2  are 
adjusted  to  the  same 
degree  of  sensitiveness, 
their  currents  will  act 
equally  and  oppositely 
upon  the  telephone  P, 
and  no  signals  will  result. 
If,  however,  D-2,  let  us 
say,  is  adjusted  to  a  high 
degree  of  sensibility  and 
D-l  to  a  lesser  degree, 
signals  will  be  received. 

Under     these     condi- 


at  C-l,  the  opposed  crystals  at   D-l  and   D-2,  the  potentiometers 


20- 


16- 


CHANGE  OF  CURRENT  IN    D-2    FOR   NORMAL  SIGNAL 


IN    D-l      FOR    NORMAL    SI6NAI 


CURRENT  OF   B   OPPOSES  CURRENT  OF 


199 — Curve 


VOLT  5 
Showing   Principle 


of    Balanced    Crystal    Receiver. 


tions,  if  an  extra  severe  discharge  of  static  passes  through  the  aerial  circuit,  almost  equal 
effects  are  produced  in  the  head  telephone  P  by  both  crystals,  and  the  crashing  sounds  ordi- 
narily experienced  partially  eliminated. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


173 


The  real  effect  of  the  balanced  crystal  receiver  is  to  limit  the  sound  produced  in  the  head 
telephones,  advantage  being  taken  of  the  peculiarities  of  the  volt-ampere  characteristic  curve. 
This  can  best  be  explained  by  the  curve,  Fig.  199,  where  the  point  X  represents  the  adjust- 
ment of  the  potentiome- 
ter P-2  for  the  crystal 
D-2,  that  is,  it  is  the 
point  where  the  addition 
of  a  slight  antenna  E. 
M.  F.  will  cause  the  cur- 
rent in  the  head  tele- 
phone circuit  to  increase 
to  the  point  Y.  On  the 
other  hand,  point  X1 
(lower  down  on  the 
curve)  is  the  adjust- 
ment corresponding  'to 
the  crystal  D-l,  where 
it  will  be  seen  that  the 
same  value  of  antenna 
voltage  which  will  cause 
a  large  increase  of  cur- 
rent in  detector  D-2  will 
cause  but  a  small  in- 


Fig.   200a— Type   I   Aerial   Changeover   Switch. 


CONNECTOR  TO  OSCILLATION 
TRANSFORMER  AND  AERIAL 


crease  of  the  current  through  the  local  circuit  of  detector  D-l.    The  current  corresponding  to 
D-l,  is  shown  at  Y1  on  the  curve  (which  is  really  a  negative  current). 

Hence  the  current  of  crystal  D-2  predominates,  and  although  signals  will  be  received 
from  any  given  station,  they  will  not  equal  in  strength  the  signal  to  be  obtained  by  the  use 
of  one  crystal. 

If  an  extra  severe  discharge  of  static  strikes  the  receiving  aerial,  due  to  its  extraordinary 
intensity,  the  current  in  the  local  circuit  of  detector  D-l  reaches  approximately  the  same  maxi- 
mum as  in  D-2,  or,  in 
other  words,  the  current 
increases  to  approxi- 
mately point  Y  on  the 
curve,  but  the  current  of 
D-l  opposes  that  of  D-2, 
and  hence  both  detectors 
act  almost  equally  and 
oppositely  on  the  receiv- 
ing telephone,  and  the 
crash  of  the  static  dis- 
charge will  be  largely 
annulled.  The  intensity 
of  strong  radio  signals 
will  be  reduced  in  the 
same  manner. 

The  adjustment  for  ob- 
taining the  best  operat- 
ing characteristic  of  this 
combination  is  readily 
obtained  by  a  skilled  op- 
erator. First,  the  sensi- 
tive spots  on  crystals  D-l 
and  D-2  are  located,  fol- 

big.    200b — Type    I    Aerial    Changeover    Switch.  ,  ,    ,  ,. 

lowed   by  corresponding 
variation  of  the  sliding  contacts  of  the  potentiometers  P-l  and  P-2. 

155.  Type  I  Aerial  Changeover  Switch. — A  plan  view  and  side  elevation  of 
this  switch  appear  in  Figs.  200-a  and  200-b,  respectively.  Several  steel  discs,  B, 
which  make  connection  with  the  spring  contacts  C  on  either  side  of  the  rod,  are 
mounted  on  a  hard  rubber  rod  R.  When  the  handle  A  is  thrown  downward  a  long 


CIRCUIT  TO  TRANSFORMER 
/  PRIMARY    CLOSED  HERE 
I 
\ 


\ 


\ 


174 


PRACTICAL  WIRELESS  TELEGRAPHY. 


ANTENHA  SWITCH 
TYPE  H* 1 


steel  bar  F  closes  the  circuit  to  the  primary  winding  of  the  power  transformer 
through  the  contacts  11  and  12.  In  the  opposite  position  the  bar  separates  the 
spring  contacts  M,  M1  200-a,  which  disconnects  the  transmitting  oscillation  trans- 
former from  the  aerial  during  the  pe- 
riod of  reception,  preventing  the  in- 
coming oscillations  leaking  to  earth. 
The  various  circuits  closed  by  the 
discs  mounted  on  the  hard  rubber  rod 
place  the  detector  and  head  telephone 
on  short  circuit  during  the  period  of 

•     •  +   n  T/f  jMjJL  \ f  transmission.     They  may  also  be  em- 

'       jjj"|^m»||         ^  ployed  to  start  and  stop  a  motor  blozver 

I  I      14—         3  i  I  or  an  automatic  motor  starter.     The 

^  ' <  f  type  I  switch  is  supplied  with  Marconi 

transmitting  sets  up  to  5  K.  W.  ca- 
pacity. 

For  the  use  of  operators  in  the  Marconi 
service,  the  circuit  diagrams,  Figs.  201,  202 
and  203,  are  published,  showing  the  con- 
nections in  Fig.  201  of  the  type  I  antenna 
switch  to  the  external  binding  posts  of  the 
type  106  receiving  tuner,  and  in  Fig.  202 
the  connections  of  the  type  S,  H,  aerial 
changeover  switch  to  the  external  binding 
posts  of  the  type  106  tuner.  The  diagram, 
Fig.  203,  shows  the  connections  of  the  type 
I  antenna  switch  to  the  external  binding 
posts  of  the  type  107-a  receiving  tuner. 

In  the  diagram,  Fig.  201,  binding  posts 
2,  4  and  5  of  the  type  I  antenna  switch 
short  circuit  the  crystal  detector  and  the 

Tune?!**  head  telephones  during  transmission.   Bind- 

ing post  No.  1  connects  to  aerial  binding 
post  No.  1  of  the  type  106  tuner.  Contacts  11  and  12  close  the  circuit  to  the  primary  winding 
of  the  transformer.  The  binding  posts  6,  7,  8  and  9  of  the  type  106  tuner  are  connected  to 
the  four  binding  posts  of  the  battery  box  as  shown. 

In  the  diagram,  Fig.  202,  posts  1,  2,  4  and  5  of  the  type  S.  H.  antenna  switch  lead  to 
contacts  which  place  the  head  telephone  and  detector  of  the  type  106  tuner  on  short  circuit. 
Binding  posts  11  and  12  close  the  circuit  to  the  primary  winding  of  the  transformer. 

In  the  diagram,  Fig.  203,  posts  2,  4  and  5  of  the  type  I  switch  lead  to  contacts  which  short 
circuit  the  receiving  detector  and  the  receiving  telephones.     These  diagrams  should  be  care- 
fully studied  and  in  event  that  the  wiring  is 
taken  down,  the  connections  shown  should 
be  carefully  duplicated. 

156.  Type  112  Receiving  Tuner. 

— This  receiving  tuner  was  pri- 
marily designed  for  use  with  the 
special  l/%  K.  W,  cargo  type  transmit- 
ter developed  by  the  Marconi  Com- 
pany and  fundamentally  is  similar*  to 
'the"  type  101  and"  type  '106  tuner,  but 
the  construction  is  considerably  simpli- 
fied. A  front  view  is  shown  in  Fig. 
203-a  and  a  side  view  in  Fig.  203-b. 
As  will  be  seen  in  the  wiring  diagram, 
Fig.  203-c,  the  circuits  are  similar  to  those 
shown  in  Fig.  153-b,  the  potentiometer, 
battery  and  head  phones  .being  shunted 


ANTENNA 
SWITCH 
TYPE'SH" 


TO   KEY 

TO  CHARGING   PANEL 


Fig.   202 — Type  S   H  Switcn  Connected   to  Type    106 
.  .    '        Receiving    Tuner. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


175 


around  the  fixed  condenser.  A  buzzer  test  circuit  is  placed  in  inductive  relation  to  the 
earth  lead  and  the  buzzer  is  thrown  into  operation  by  means  of  a  small  push  button  mounted 
on  the  front  of  the  panel. 

The  receiving  transformer  is  of  the  inductively  coupled  type  consisting  of  a  two-layer 
primary  and  a  two-layer  secondary  winding,  the  inductance  of  the  primary  winding  being 
regulated  by  a  "tens"  switch  and  a  "units"  switch.  The  "unit"  switch  cuts  in  ten  single 
turns  in  the  primary  winding  and  the  "tens"  switch  ten  turns  at  a  time 

The  inductance  of  the  secondary  winding  is  altered  by  a  simple  multi-point  switch. 
The  secondary  shunt  condenser  consists  of  two  concentric  brass  tubes,  which  are  telescoped 
by  a  small  knob  in  the  slot  on  the  top  of  the  panel. 


l"l,  TUNE  Q 


2    CELLS 
DRY  BATTERY 


TO  CHARGING    PANEL  ^"  • 

Fig.    203 — Type   I   Switch    Connected   to   Type    107a   Receiving   Tuner. 

The  coupling  between  the  primary  and  secondary  windings  is  altered  by  a  knob  sliding 
in  the  lower  slot  to  the  front  of  the  panel.  When  the  knob  is  placed  to  the  extreme  left 
the  coupling  is  maximum,  lesser  degrees  being  obtained  by  moving  this  knob  in  the  opposite 
direction. 

The  series  antenna  condenser  is  thrown  in  the  aerial  circuit  by  means  of  the  switch 
marked  "condenser-in-out."  This  condenser  is  of  fixed  value,  the  necessary  adjustment 
for  resonance  being  obtained  at  the  primary  inductance. 

The  type  112  tuner  is  fitted  with  a  carborundum  detector,  the  latter  being  mounted 
immediately  to  the  front  of  the  panel.  The  local  battery  current  for  the  crystal  is  turned 
on  and  off  at  the  switch  marked  "battery-on-off."  This  switch  should  always  be  in  the 
"off"  position  when  the  receiver  is  not  in  use. 


176 


PRACTICAL  WIRELESS  TELEGRAPHY. 


A  sensitive  spot  is  found  on  the  crystal  by  pressing  the  push  button  and  "feeling  about" 
on  the  crystal  with  the  pointed  contact  until  the  loudest  response  is  obtained,  simultaneous 
adjustment  being  made  of  the  potentiometer. 

The  receiving  transformer  is  adjusted  to  the  incoming  signal  as   follows: 

(1)  Adjust  detector  by  buzzer  tester. 

(2)  Place  the  coupling  knob  as  far  to  the  left  as  possible. 

(3)  Vary   inductance    of  the  primary   and   secondary   windings  progressively 
until  the  desired  station  is  heard. 

(4)  When  signals  are  heard  decrease  the  coupling  slightly. 

(5)  Then  retune  primary  and  secondary  circuits,  using  slight  values  of  capacity 
at  the  secondary  condenser. 

(6)  Then  slightly  vary  the  coupling  until  loud  signals  are  obtained  or  until 
an  interfering  signal  is  eliminated. 


KTICTOR 


5UZ7EK  TESTER 


fcf— il 


•o*  * 


POTENTIOMETER 


Fig.   203a — Front  View   of  the   Marconi    Type    112    Receiving  Tuner    (The  American    Marconi  Company). 

156a.  General  Advice  for  the  Manipulation  of  a  Receiving  Tuner. — 
While  learning  to  manipulate  a  receiving  tuner  of  any  type,  the  student  should 
first  study  a  complete  fundamental  wiring  diagram  of  the  set  and  note  the  corre- 
sponding parts  on  the  actual  equipment.  He  should  observe  particularly  the 
direction  in  which  the  inductance,  changing  switches  and  the  variable  condenser 
control  knobs  are  moved  for  an  increase  of  their  values.  It  should  always  be 
borne  in  mind  that  any  increase  or  decrease  of  wave  length  adjustment 
in  the  antenna  circuit  should  be  followed  by  a  similar  increase  or  decrease  in 
the  secondary  circuit.  Also  a  change  of  coupling  slightly  influences 
the  effective  self-inductance  of  the  primary  and  secondary  windings,  and  therefore 
if  the  inductance  of  either  winding  is  adjusted  for  a  given  wave  length,  and  after- 
ward the  coupling  decreased  to  minimize  interference,  a  new  value  of  inductance 
must  be  selected  at  the  primary  and  secondary  coils ;  or,  if  the  period  of  these 
circuits  is  varied  by  means  of  variable  condensers,  a  slight  change  in  capacitance 
should  be  made. 


RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS. 


177 


(a)  It  should  be  remembered  in  adjusting  a  station  for  the  first  time  that  the  adjust- 
ments  of  any  given  tuner  will  vary  with  the  different  aerials.     If  the  tuner  is  also 
strange  to  the  operator,  the  best  method  to  pursue  is  to  tune  first  the  standard  600 
meter  wave,  using  a  rather  large  amount  of  inductance  and  relying  upon  the  series 
condenser  to  vary  the  antenna)  wave  length  to  pick  up  signals,  thereafter  reducing  the 
inductance  and  eliminating  the  short  wave  condenser  until  the  maximum  strength  of 
signals  is   obtained. 

(b)  If  a  call  is  expected  from  one  of  several  wireless  stations  not  accurately  tuned 
to  the  same  wave  length,  close  coupling  should  be  employed  between  the  primary  and 
secondary  windings  and  the  shunt  secondary  condenser  set  at  zero,  but  after  communi- 
cation   is    established,    lesser    values    of    coupling    should    be    employed    to    prevent 
interference.      (See   Section   H   appendix.) 


Fig.    203b—  Rear    View    of    Marconi    Type    112    Receiving    Tuner    (The    American    Marconi    Company). 

(c)  Due  to  the  fact  that  the  wave  length  of  the  average  ship's  aerial  lies  between 
300  and  600  meters,  only  a  few  turns  of  inductance  are  required  to  adjust  the  antenna 
system  to  the  wave  length  of  600  meters.     On  the  other  hand,  the  short  wave  variable 
condenser  must  invariably  be  employed  to  obtain  the  300  meter  wave  with  a  ship's 
aerial. 

(d)  If  the  tuner  is  fitted  with  a  primary  shunt  condenser,  increased  selectivity  may 
be    obtained   by   proper   selection   of   capacity   values,   but   generally   with    decreased 
strength  of  signals. 

(e)  Severe  discharges  of  atmospheric  electricity  may  temporarily  or  permanently 
destroy  the  sensitiveness  of  a  receiving  detector,  hence  frequent  readjustment  is  neces- 
sary during  an  operating  schedule.     These  remarks,  however,  do  not  apply  to  a  highly 
exhausted  three-element  vacuum  valve,  the  magnetic  detector,  the  Fleming  oscillation 
valve,  or  others  which  possess  a  marked  degree  of  stability. 

(f)  Frequent  use  of  the  test  buzzer  will  indicate  whether  or  not  the  crystal  detector 
is  in  adjustment.     It  has  been  observed  that  the  adjustment  of  the  crystal  detector 
giving  good  response  to  a  buzzer  is  not  necessarily  the  most  sensitive  adjustment  for 
a  far  distant  station,  and  similarly  with  most  types  of  receiving  detectors,  the  adjust- 


178 


PRACTICAL   WIRELESS   TELEGRAPHY. 


ment  giving  maximum  response  to  nearby  stations,  is  not  the  most  sensitive  adjust- 
ment for  a  far  distant  transmitting  station. 

(g)  Not  all  carborundum  crystals  possess  equal  degrees  of  sensitiveness;  conse- 
quently those  giving  maximum  response  to  far  distant  stations  should  be  used. 

(h)  The  maximum  degree  of  selectivity  is  obtained  in  receiving  circuits  of  radio- 
frequency  when  the  loosest  coupling  which  will  give  a  readable  signal  is  employed 
and  when  inductance  is  used  to  tune  aerial  circuits  and  some  capacity  is  used  in 
shunt  to  the  secondary  coil. 


WIRES  1,1,4,5    CONNECT  TO  TYPE  I  AERIM.  SWITCH 

Fig.    203c— Complete   Wiring   Diagram    of   the  Type    112    Receiving  Tuner    (The   American    Mar- 
coni   Company). 

(i)  The  diaphragm  of  receiving  telephones  should  frequently  be  removed  and 
carefully  wiped  to  prevent  the  formation  of  rust.  Bent  diaphragms  should  be  imme- 
diately replaced  by  new  ones.  The  diaphragm  should  not  touch  the  surface  of  the 
magnet. 

(j)  If  in  doubt  concerning  the  continuity  of  the  circuits  of  a  receiving  tuner,  a 
testing  circuit  comprising  a  dry  cell  battery  and  a  head  telephone  connected  in  series 
should  be  applied  to  the  primary  and  secondary  coils  to  ascertain  if  the  circuit  is  in 
conductive  condition. 


PART  X. 

AUXILIARY  APPARATUS  OR  EMERGENCY 

TRANSMITTERS. 

157.  STATUTE  REQUIREMENTS.  158.  TUNED  COIL  SET.  159. 
THE  ELECTRIC  STORAGE  BATTERY  COMPANY'S  ACCUMULATORS 
AND  CHARGING  PANEL. 

157.  Statute  Requirements.— The  United  States  Statute  (Act  of  August  13, 
1912),  requires  that  all  vessels  carrying  radio  equipment  be  fitted  with  an  emer- 
gency transmitter  which  can  be  operated  independently  of  the  ship's  generator. 
This  sender  must  have  a  daylight  transmission  range  of  at  least  100  miles,  and 
must  be  capable  of  functioning  continuously  for  a  period  of  four  hours.  If  an 
independent  emergency  transmitter  is  not  supplied,  a  source  of  auxiliary  power 
of  sufficient  capacity  to  operate  the  motor  generator  independently  of  the  ship's 
generator  must  be  available.  The  regulations  of  the  International  Radio  Tele- 
graphic Convention  require  the  emergency  apparatus  to  be  capable  of  functioning 
for  at  least  six  hours,  and  to  have  a  minimum  range  of  80  nautical  miles  in  case  of 
a  ship  having  constant  radio  service,  and  50  miles  where  the  ship's  station  has  a 
service  of  limited  duration. 

Certain  vessels  of  the  American  Marconi  Service  are  fitted  with  a  60  cell  stor- 
age battery  of  capacity  varying  from  60  to  224  ampere  hours.  The  batteries  of 
lower  capacity  simply  furnish  current  for  the  operation  of  the  motor  generator,  but 
those  of  higher  capacity  are  employed  to  operate  a  few  emergency  lights  as  well. 
The  60  cell  battery  generally  the  property  of  the  Steamship  Company  is  under  the 
direct  supervision  of  the  Chief  Engineer,  but  the  operators  of  the  Marconi  Com- 
pany must  have  sufficient  knowledge  of  the  circuits  and  the  manipulation  of  the 
charging  panel  to  keep  the  batteries  in  a  fully  charged  condition. 

If  for  any  reason,  a  60  cell  battery  is  not  supplied,  an  emergency  transmitter 
developed  by  the  Marconi  Company  is  employed.  This  sender  consists  of  a  10- 
inch  induction  coil  operated  by  a  30-volt  storage  battery,  the  induction  coil  being 
employed  to  excite  the  condenser  of  the  power  set  in  place  of  the  usual  high  voltage 
transformer. 

The  auxiliary  transmitter,  originally  planned  by  the  Marconi  Company,  is 
shown  in  Fig.  204,  where  the  spark  gap  of  a  10-inch  induction  coil  is  connected 
directly  in  series  with  the  aerial  system,  the  primary  winding  being  energized  by 
a  storage  battery  of  24  to  30  volts.  Provision  was  made  on  some  ships  whereby 
these  coils  could  be  operated  either  directly  from  110  volts  D.  C.,  with  a  special 
rheostat  connected  in  series  or  from  the  storage  battery. 

The  apparatus  shown  in  the  circuit  of  Fig.  204  has  been  termed  the  (tplain 
aerial  set"  because  the  spark  gap  is  connected  directly  in  series  with  the  antenna 
circuit.  The  use  of  the  plain  aerial  transmitter  is  not  allowed  by  the  United  States 
regulations  except  in  case  of  emergency,  because  when  no  extra  inductance  is 
inserted  at  the  base  of  the  antenna  system,  a  highly  damped  wave  is  radiated 
which  will  interfere  with  the  dispatch  of  traffic  between  other  stations. 


180 


PRACTICAL   WIRELESS   TELEGRAPHY. 


158.  Tuned  Coil  Set. — When  the  induction  coil  is  employed  to  excite  the 
closed  circuit  condenser  as  in  Fig.  205,  it  is  called  the  "tuned  coil  set."  The  great 
advantage  of  this  connection  lies  in  the  fact  that  the  antenna  oscillations  will  be 
damped  out  less  rapidly  than  with  the  "plain  aerial"  connection,  and  the  radiated 
wave  will  cause  less  interference  to  the  operation  of  other  stations. 

A  description  of  the  circuits  follows :  The  apparatus  of  the  power  set  is  represented  by  the 
alternator  armature  A,  C,  the  primary  winding  of  the  closed  core  transformer  P,  the  circuit 
including  the  wattmeter  W.  The  secondary  circuit  comprises  the  secondary  winding  of  the 
transformer  S,  the  high  potential  condenser  C-l,  spark  gap  G  and  the  primary  winding  of  the 
oscillation  transformer  P-l.  The  aerial  circuit  is  represented  by  the  secondary  winding  S-l, 
the  short  wave  condenser  C-2  and  the  aerial  tuning  inductance  S-2. 

The  circuit  of  the  emergency  induction  coil  extends  from  the  D.  C.  switch  at  the  left  of 
the  drawing,  though  the  underload  circuit  breaker  B,  through  the  regulating  resistance  R,  to 
a  24-30-volt  storage  battery  shunted  by  a  small  voltmeter  V.  The  charging  resistance  R  is 
proportioned  to  pass  from  6  to  7  amperes  through  the  storage  battery  during  the  charging 
process.  The  function  of  the  underload  circuit  breaker  has  been  described  in  Part  V,  and 
it  is  sufficient  to  say  here  that  it  automatically  opens  the  circuit  in  case  the  voltage  of  the 
generator  falls  below  that  of  the  storage  battery. 


Z4VOLT   STORAGE 
BATTERY 


6V. 


6V. 


Fig.    204 — "Plain    Aerial"    Emergency    Transmitter. 


(a)  Adjustment  and  Timing  of  the  Set.  To  place  the  storage  battery  on 
charge,  the  D.  C.  switch  is  closed  and  the  plunger  U  of  the  underload  circuit 
breaker  pushed  upward  by  hand,  the  charging  circuit  thereby  being  closed  through 
the  contact  points  E,  E. 

The  circuit  from  the  battery  to  the  induction  coil  may  be  traced  through  the 
telegraph  key,  through  the  primary  winding  of  the  induction  coil  P-3,  and  thence 
through  the  interrupter  I,  the  vibrator  condenser  being  represented  at  K.  In 
order  that  a  uniform  spark  discharge  may  be  obtained,  careful  adjustment  must 
be  made  of  the  stationary  contact  of  the  interrupter  I  until  a  discharge  free  from 
arcing  is  secured,  but  generally  at  its  best  the  pitch  of  the  note  produced  by  these 
coils  will  be  rather  low. 

In  case  of  accident  to  the  power  apparatus,  such  as  the  burning  out  of  the  motor  generator 
or  the  high  potential  transformer,  the  emergency  transmitter  is  put  into  action  in  the  follow- 
ing manner:  The  switch  H-2  is  placed  in  the  downward  position.  This  connects  the  tele- 
graph key  in  series  with  the  primary  winding  of  the  induction  coil.  Next  the  plug  contact 


AUXILIARY  APPARATUS  OR  EMERGENCY  TRANSMITTERS. 


181 


R-4  is  removed  from  the  connector  R-l  and  plugged  into  R-3,  thereby  connecting  the  secon- 
dary winding  of  the  induction  coil  to  the  condenser  C-l.  (A  single  blade  double-throw  high 
voltage  switch  is  often  used  instead  of  the  flexible  plug  connector.)  After  these  connections 
have  been  made,  the  telegraph  key  is  pressed  and  the  vibrator  of  the  induction  coil  adjusted 
simultaneously  until  uniform  interruptions  are  secured.  Generally  with  this  connection,  it 
becomes  necessary  to  reduce  the  length  of  the  spark  gap  G,  and  in  case  a  quenched  spark  gap 
is  employed,  to  use  no  more  than  two  or  three  plates.  If  a  rotary  gap  is  included  in  the  power 
transmitter,  it  must  be  stopped  with  the  spark  electrodes  in  such  position  as  will  afford  a  dis- 
charge gap  of  the  correct  length  for  the  coil.  Since  the  closed  and  open  oscillation  circuits 
of  the  transmitter  are  already  adjusted  to  resonance,  the  complete  process  of  tuning  need 
not  be  gone  over  again,  with  the  exception  that  the  coupling  between  the  primary  winding 
P-l  and  the  secondary  winding  S-l  may  have  to  be  increased. 

The  ammeter  connected  in  the  antenna  circuit  of  this  set  should  read  from  2  to  3  amperes, 
depending  upon  the  electrical  dimensions  of  the  circuits  of  radio-frequency,  or,  in  other  words 
the  type  of  set.  The  range  of  transmission  varies  from  125  to  225  miles  in  daylight. 


AUXILIARY  TRANSMITTER 
CIRCUIT 


Fig.    205 — Tuned    Coil    Emergency    Transmitter. 


The  coil  transmitters  are  generally  supplied  with  the  portable  chloride  type 
D-5  accumulator  of  the  Electric  Storage  Battery  Company,  but  later  installations 
are  equipped  with  another  type  known  as  the  5  K.  X.  portable  accumulator,  either 
battery  being  of  60  ampere  hours  capacity. 

The  general  instructions  given  for  the  chloride  battery  in  Part  V,  apply  to  the 
batteries  of  these  sets,  and  should  be  carefully  observed.  It  will  be  noted  from 
the  diagram  of  Fig.  205  that  the  battery  can  be  placed  on  charge  with  the  induc- 
tion coil  in  operation.  As  a  safety  measure,  to  prevent  the  meter  being  burned 
out  by  electrostatic  induction  from  the  transmitter,  the  voltmeter  circuit  contains 
a  small  strap  key,  which  must  be  closed  to  take  a  reading  of  the  voltage  of  the  cells. 

159.  The  Electric  Storage  Battery  Company's  Accumulators  and  Charging 
Panel. — The  type  and  rating  of  the  Exide  cells  furnished  by  the  Electric 


182  PRACTICAL  WIRELESS  TELEGRAPHY. 

Storage   Battery   Company   for   auxiliary   power  is   shown  in   the   following 
table  :— 

TYPE  RATING 

60  cell    7  PV —  72  ampere  hours; 

60  cell  11  NV—  140 

60  cell  13  MV—  168 

60  cell  17  MV—  224 

Type  7  PV  battery  is  employed  in  connection  with  the  ^2 -kilowatt  panel  trans- 
mitting set,  type  P-5  of  the  Marconi  Company.  The  other  types  are  for  opera- 
tion of  the  2-kilowatt,  500-cycle  motor  generators  furnished  with  Marconi  type 
P-4  set  and  also  for  auxiliary  light  aboard  the  vessel. 

A  photograph  of  the  E.  S.  B.  Co.'s  charging  panel  is  shown  in  Fig.  206,  and 
a  fundamental  diagram  of  the  connections  from  the  board  to  the  batteries  and 
motor  generator  in  Fig.  207.  The  latter  diagram  includes  the  complete  connec- 
tions of  the  Marconi  2-K.  W.  500-cycle  Crocker-Wheeler  type  of  motor  generator, 
and  the  automatic  starter.  Fig.  208  shows  a  rear  view  of  the  wiring  of  the  panel. 


Fig.   205a— Charging  Panel  Used   with   Marconi   Tuned   Coil    Sets. 

In  the  diagram  of  connections,  Fig.  207,  the  connections  for  the  voltmeter  and 
the  double-pole  double-throw  switch  and  other  switches  for  auxiliary  lighting 
service  are  purposely  left  out  to  simplify  the  drawing,  but  a  detail  of  the  volt- 
meter connection  appears  in  Fig.  208. 

Returning  to  the  diagram  of  Fig.  207:  All  apparatus  concerned  in  the  charge  and  dis- 
charge of  the  batteries  appear  to  the  left  of  the  drawing,  and  the  circuits  of  the  automatic 
starter  to  the  right,  including  the  wiring  of  the  motor  generator.  A  six-pole  double-throw 
switch,  with  knife-blades  1,  :2,  3,  4,  5,  6,  when  thrown  up  in  the  drawing  or  to  the  left  on  the 
actual  charging  panel,  places  battery  units  A  and  B  on  charge,  but  in  the  opposite  position,  on 
discharge],  the  circuits  from  the  battery  continuing  through  the  blades  5  and  6  to  the  auto- 
matic starter  and  motor  generator. 

The  resistance  coils  R-l,  R-2  and  R-3  are  connected  in  the  charging  circuit  of  battery  A, 
through  the  knife  blades  1  and  2  and  coils  R-4,  R-5  and  R-6,  through  the  blades  3  and  4  to 
battery  B.  The  overload  circuit  breaker  K  is  connected  in  series  with  the  common  charging 
lead  of  battery  units  A  and  B.  The  winding  of  the  no-voltage  release  magnet  for  this  breaker 
indicated  at  N  has  the  series  protective  resistance  R-l.  Contacts  C-l  and  C-2  mounted  on 


AUXILIARY  APPARATUS  OR  EMERGENCY  TRANSMITTERS.  183 


LEAK  LAMP 


VOLTMETER 
SWITCH 


AMPERE  HOUR 
METER 


POLARITY  REVERSING 
-      SWITCH 


CHARGING 
POSITION 


L1GHT5  FROM 
GENERATOR 


UNDERLOAD  AND 
ERLOAD  BREAKER 


_VOLTMETER 


_J>I5CHARGE  SIDE 
OF  SWITCH 


LIGHTS   FROM 
"""BATTERY 


Fig.  206 — Standard  Charging  Panel  Supplied  by  the  Electric  Storage  Battery  Company  for  Marine  Use. 


184 


PRACTICAL   WIRELESS   TELEGRAPHY 


wwwvw- 

c£ 

-HWWWVW-h 


AUXILIARY  APPARATUS  OR  EMERGENCY  TRANSMITTERS. 


185 


the  circuit  breaker  open  the  circuit  of  the  no-voltage  release  magnet  when  the  breaker  trips. 
It  should  be  thoroughly  understood  that  this  breaker  will  open  the  charging  circuit  in  case 
of  an  overload  during  the  charging  period,  or  in  case  the  voltage  of  the  generator  falls  below 
the  voltage  of  the  battery.  The  breaker  will  also  open  when  the  pointer  on  the  ampere-hour 
meter  touches  the  contact  point  at  the  zero  position  (during  the  charging  process). 


CHARGING  RESISTANCES 
FOR  BATTERY  "A" 


CHARGING    RESISTANCES 
FOR  BATTERY   '&" 


AUX  OPINING  SW. 
OPENS  WHEN. 
CB.  OPENS 
RESISTANCE 
LOW  VOLTAGE 


Fig.  208 — Rear  View  Actual   Wiring  Diagram   of  Electric  Storage   Battery  Company's 

Charging    Panel. 

P-l  and  P-2  are  25-mzff  incandescent  lamps  which  are  shunted  around  the  circuit 
breaker  and  the  resistance  units.  Through  these,  a  small  value  of  charging  current  con- 
stantly flows  through  the  cells,  compensating  for  local  losses,  such  as  may  be  caused  by 
local  action  or  a  slight  surface  leakage  of  the  cell. 


186 


PRACTICAL   WIRELESS   TELEGRAPHY. 


TO  DISCHARGE  MAINS 
GO  CELLS  IN  SERIES 


An  important  part  of  this  apparatus  is  the  double  pole  double  throw  reversing  switch 
placed  in  the  upper  left-hand  corner  of  the  drawing.  Should  the  connections  from  the 
ship's  generator  to  the  switchboard  be  reversed,  say,  during  repair  or  rewiring,  the  correct 
polarity  can  always  be  obtained  by  plugging  in  the  voltmeter  connection  to  the  power  line, 
followed  by  closing  this  switch.  If  the  pointer  of  the  meter  moves  in  the  proper  direction 
or  across  the  scale,  the  polarity  of  the  charging  mains  is  correct.  If  the  pointer  moves  in 
the  wrong  direction,  the  double-pole  switch  must  be  thrown  in  the  opposite  direction.  Pro- 
tective fuses  are  inserted  in  the  battery  charge  and  discharge  circuits  to  open  the  circuit  in 
case  of  overload. 

The  general  operating  instructions  for  the  Exide  cells  given  in  Part  V  should 
have  careful  attention.  The  operator  should  aim  to  keep  the  batteries  in  a  fully 
charged  condition  at  all  times,  or  confining  the  remarks  to  daily  practice,  the 
pointer  of  the  amperehour  meter  should  always  be  near  to  the  zero  position  and 
if  after  use  of  the  battery  the  pointer  indicates  that  a  given  quantity  of  current 
has  been  taken  out,  it  should  be  placed  immediately  on  charge  and  allowed  to 
remain  so  until  the  pointer  returns  to  the  zero  position  of  the  scale  of  the  ampere- 
hour  meter. 

One  part  of  this  circuit  may  require  explanation.  It  will  be  observed  that  the  ampere- 
hour  meter  is  merely  connected  in  series  with  one  of  the  battery  units  (Battery  A)  when 

on  charge.  The  neces- 
sity for  this  connection 
will  be  evident  from  the 
following  explanation : 
If  the  meter  were  con- 
nected in  series  with 
both  battery  units,  the 
pointer  of  the  ampere- 
hour  meter  would  travel 
across  the  scale  at  dou- 
ble the  rate  of  the  actual 
charging  current  flowing 
to  each  battery  unit  for 
the  reason  that  the  bat- 
teries are  connected  in 
parallel.  As  an  exam- 
ple, if  the  amperehour 
meter  were  connected  in 
series  with  both  battery 
groups  on  charge  and 
further  that  a  quantity 
equal  to  ten  ampere 
hours  of  current  passed 
from  the  generator  to 
the  battery,  each  set  of 
cells  actually  would  re- 
ceive but  five  amperehours  of  the  charging  current,  or,  in  other  words,  the  meter  would 
indicate  a  value  actually  double  the  charging  current  supplied  to  each  group  of  cells.  Hence 
the  amperehour  meter  is  simply  connected  in  series  with  one  unit  on  charge,  but  in  series 
with  both  units  on  discharge,  the  assumption  being  made  that  the  two  battery  units  are  in 
identical  states  of  charge.  Any  dissimilarity  in  the  two  groups  of  cells  can  easily  be  de- 
termined by  the  readings  of  the  voltmeter  during  load  and  proper  correction  made  therefor. 
In  actual  commercial  practice,  no  difficulty  is  found  in  keeping  both  battery  units  in  the 
same  state  of  charge  from  month  to  month. 

A  detailed  diagram  of  the  voltmeter  plug  switch  is  shown  in  Fig.  209.  It  will  be  noted 
that  a  small  push  button  key,  connected  in  series  with  the  voltmeter,  must  be  ^  closed  in 
order  to  take  the  reading.  A  small  resistance  or  multiplier  is  connected  in  series  as  the 
drawing  indicates. 

Plugs  1  and  2  connect  to  the  terminals  of  the  60-cell  battery  with  all  cells  connected  in 


^H 

\ 

ACROSS    SHIPS 
-    GENERATOR 

f        t 

f 

^ 

] 

i 
7 

\ 

»        < 

5 

!                                          1 

6 

BATTERY 

UNIT  "B" 

DETAIL    VOLTNAETEr\   CONNECTION 
E.S.B.Co.  CHARGING  PANEL 

Fig.    209 — Detail    Voltmeter    Connection,    Electric    Storage    Battery    Com- 
pany's   Charging   Panel. 


AUXILIARY  APPARATUS  OR  EMERGENCY  TRANSMITTERS.  187 

series.  Plugs  3  and  4  connect  across  the  terminals  of  battery  unit  A.  Plugs  5  and  6  con- 
nect to  the  mains  from  the  ship's  generator.  Plugs  7  and  8  connect  to  the  terminals  of 
battery  unit  B. 

Although  the  circuits  of  the  motor  generator  and  automatic  starter  indicated  in  Fig.  207 
have  been  gone  over  in  the  chapter  on  motor  generators,  the  notations  in  Fig.  207  for  the 
same  circuits  will  now  be  explained. 

S-l  is  the  motor  starting  switch  which  may  be  closed  by  auxiliary  contacts  on  the  aerial 
changeover  switch  or  by  a  small  push  button  switch  near  to  the  transmitting  key  of  the 
operator.  R-3  is  the  fixed  resistance  in  series  with  the  automatic  starter  winding  W-l, 
which  is  thrown  in  the  circuit  when  the  starter  is  in  the  full  running  position,  that  is  to 
say,  it  is  connected  in  by  the  upward  movement  of  the  plunger.  Resistance  R-2  is  connected 
in  series  with  the  relay  winding  W-2  to  prevent  an  excessive  flow  of  current.  Relay  wind- 
ing W-3  is  connected  in  series  with  the  motor  armature  and  in  case  of  an  overload,  auto- 
matically opens  the  solenoid  winding  W-l  through  the  contacts  C-5  and  C-6.  R-4  is  the 
electrodynamic  brake  resistance  thrown  in  shunt  to  the  armature  when  the  plunger  of  the 
automatic  starter  drops  downward. 

F-l  is  the  shunt  field  of  the  motor,  the  regulating  rheostat  R-5  being  connected  in  series. 
F-2  is  the  generator  field  winding  with  the  regulating  rheostat  R-6  connected  in  series. 
This  circuit  is  closed  by  the  automatic  starter  at  point  5,  or,  in  other  words,  current  does 
not  flow  through  the  generator  field  winding  until  the  motor  has  attained  normal  speed. 

Jt  is  believed  that  with  the  information  supplied  in  this  and  Chapter  5  at 
hand,  commercial  operators  will  fully  understand  the  circuits  of  emergency 
battery  panels  and  transmitters  and  will  be  enabled  to  handle  them  successfully 
in  commercial  working. 

Vessels  fitted  with  radio  apparatus  by  affiliated  Marconi  companies  may  have 
storage  cells  and  charging  panels  which  differ  in  mechanical  construction,  but 
fundamentally  they  operate  along  the  same  general  principles.  Specific  instruc- 
tions for  each  type  of  cell  or  panel  are,  of  course,  furnished  to  marine  radio 
installations  by  the  manufacturer. 

Operators  of  the  Marconi  Service  should  observe  carefully  the  following  rules: 

(1)  Keep  battery  plates  covered  with  pure  water  at  all  times. 

(2)  Keep  the  batteries  fully  charged. 

(3)  Do  not  place  battery  on  charge  when  ship's  generator  is  not  running. 

(4)  Watch  the  polarity  of  the  charging  line   and  throw  reverse   switch  on 
charging  panels  accordingly. 

(5)  Examine  the  connections  between  all  battery  cells  frequently. 


PART  XL 
PRACTICAL  RADIO  MEASUREMENTS. 

MEASUREMENT    OF   WAVE    LENGTH— DECREMENT— CALIBRA- 
TION—TRANSMITTING  AND  RECEIVING  APPARATUS. 

160.  THE  IMPORTANCE  OF  ELECTRICAL  RESONANCE.  161.  INDI- 
CATORS OF  RESONANCE.  162.  USES  OF  THE  WAVEMETER.  163. 
SIMPLE  USE  OF  THE  WAVEMETER.  164.  GENERAL  INSTRUC- 
TIONS FOR  TUNING  A  RADIO  TRANSMITTER.  165.  TUNING  BY 
THE  HOT  WIRE  AMMETER.  166.  TUNING  THE  2  K.  W.  500 
CYCLE  PANEL  TRANSMITTER.  167.  DETERMINATION  OF  COUP- 
LING. 168.  PLOTTING  OF  RESONANCE  CURVES.  169.  MEASURE- 
MENT OF  THE  LOGARITHMIC  DECREMENT  OF  DAMPING.  170. 
CALCULATION  OF  THE  DECREMENT  OF  THE  WAVEMETER  (OR 
DECREMETER).  171.  WAVEMETER  AS  A  SOURCE  OF  HIGH  FRE- 
QUENCY OSCILLATIONS.  172.  CALIBRATION  OF  THE  SECONDARY 
AND  PRIMARY  CIRCUITS  OF  A  RECEIVING  TUNER.  173.  CALIBRA- 
TION OF  THE  OPEN  AND  CLOSED  CIRCUITS  SIMULTANEOUSLY. 

174.  MEASUREMENT  OF  THE  NATURAL  OSCILLATING  PERIOD  OF 
A  COIL.  175.  MEASUREMENT  OF  ELECTROSTATIC  CAPACITY. 
176.  MEASUREMENT  OF  THE  EFFECTIVE  INDUCTANCE  OF  A  COIL 
AT  RADIO  FREQUENCIES.  177.  CALCULATION  OF  INDUCTANCE 
FROM  THE  CONSTANTS  OF  THE  COIL.  178.  MEASUREMENT  OF 
THE  EFFECTIVE  INDUCTANCE  AND  CAPACITY  OF  AN  AERIAL. 
179.  CALIBRATION  OF  A  WAVEMETER  FROM  A  STANDARD.  180. 
MEASUREMENT  OF  MUTUAL  INDUCTANCE  AT  RADIO  FRE- 
QUENCIES. 181.  COMPARATIVE  MEASUREMENT  OF  THE  STRENGTH 
OF  INCOMING  SIGNALS.  182.  "TIGHT"  AND  "LOOSE"  COUPLING. 
183.  MEASUREMENT  OF  HIGH  VOLTAGES.  184.  TUNING  AND  AD- 
JUSTMENT RECORD. 

160.  The  Importance  of  Electrical  Resonance. — It  is  essential  that  the 
open  and  closed  oscillation  circuits  of  both  the  radio-transmitter  and  receiver  be 
substantially  adjusted  to  the  same  natural  frequency  of  oscillation.  To  place  cir- 
cuits of  radio-frequency  in  resonance  or  to  adjust  them  to  any  desired  frequency 
of  oscillation,  standard  resonating  circuits  known  as  wave  meters  are  employed. 
In  a  previous  chapter  we  have  explained  that  a  hot  wire  ammeter  may  be  employed 
for  determining  conditions  of  electrical  resonance  in  two  coupled  circuits,  but 
there  is  a  disadvantage  in  this  method  of  tuning,  for  unless  the  inductance  and 
capacity  of  each  circuit  are  known,  the  resulting  frequency  of  wave  length  corre- 
sponding thereto,  cannot  be  determined ;  hence  the  utility  of  the  wave  meter. 

A  circuit  like  that  shown  in  Fig.  210a,  consisting  of  a  radio-frequency  inductance  L  and  a 
variable  condenser  C  constitutes  an  oscillating  circuit  of  variable  frequency,  the  actual  value 
for  any  particular  setting  of  the  variable  condenser  being  determined  by  the  equation : 

5,033,000 

vTc~ 


PRACTICAL  RADIO  MEASUREMENTS. 


189 


where  L  and  C  are  expressed  in  centimeters  and  microfarads  respectively.  Hence  at  each 
division  of  the  condenser  scale,  we  might  mark  the  particular  frequency  at  which  the  circuit 
oscillates  when  set  into  excitation.  But  since  a  given  frequency  of  oscillation  corresponds  to 
a  luaic.  of  definite  length  (if  the  oscillator  were  radiative),  we  can  calibrate  the  scale  of  the 
instrument  directly  in  wave  lengths  rather  than  in  oscillation  frequencies. 


LAMP 


I.* 

0- ' 


Fig.  210a,  b,  c,  d,  e,  f,  g,  h,  i,  j — Methods  for  Determining  Resonance  Between  Wave- 
meter    and    Circuit    Under    Measurement. 

If  either  the  inductance  of  the  coil  or  the  capacity  of  the  condenser  in  the  circuit,  Fig. 
210a,  is  variable,  we  may  fit  the  variable  element  with  a  scale  of  wave  lengths  and  the  instru- 
ment will  then  be  called  a  wavemeter. 

Now  if  the  coil  L  is  placed  in  inductive  relation  to  any  part  of  an  active  oscillation  circuit, 
such  as  the  closed  or  open  circuit  of  a  radio  transmitter,  radio-frequent  oscillations  will  be 
induced  in  the  circuits  of  the  wavemeter  when  it  is  adjusted  to  the  frequency  of  the  gen- 


190 


PRACTICAL  WIRELESS  TELEGRAPHY. 


crating  circuit.  Some  indicator  of  maximum  flow  of  current  or  maximum  voltage  must  now 
be  inserted  in  the  circuit  (of  the  wavemeter)  in  order  that  the  exact  point  of  resonance  on 
the  variable  element  may  be  located. 

161.  Indicators  of  Resonance. — That  the  wavemeter  is  in  resonance  with 
the  circuit  under  measurement,  may  be  determined  by  one  of  several  instru- 
ments. 

In  Fig.  210b  a  milliammetcr  M,  range  0-200  milliamperes,  is  connected  in  series  with  the 
wavemeter  for  determining  the  adjustment  of  maximum  current  flow. 

In  Fig.  210c,  a  small  hot  wire  zvattmeter  W ,  range  .01-0.1  watts,  is  connected  to  the 
secondary  winding  S,  of  a  small  step  down  transformer,  the  primary  P  being  connected  in 
series  with  the  wavemeter. 

In  Fig.  210d,  a  small  glow  lamp  G  (2  or  4  volt  battery  lamp),  is  connected  in  series  with 
the  wavemeter,  the  resonant  adjustment  being  determined  when  tke  lamp  glows  brightest 
(with  the  wavemeter  in  a  fixed  position). 

In  Fig.  210e,  a  thermo-couple  A,  B,  is  attached  to  a  heating  wire  C,  D,  the  latter  being 
connected  in  series  with  the  wavemeter.  The  terminals  of  the  thermo-couple  are  connected 
to  a  sensitive  milli-voltmeter  which  may  be  calibrated  in  milliamperes. 

In  Fig.  210f,  a  rectifying  detector  D  is  connected  in  series  with  a  galvanometer  G,  both 
being  shunted  across  the  condenser  C.  The  currents  of  radio-frequency  are  converted  by 

the  rectifier  to  direct  current  and  the 
resonant  adjustment  is  determined  by  the 
maximum  deflection  of  the  galvanometer. 
In  Fig.  210g,  an  electrostatic  telephone  P 
is  connected  in  series  with  the  wavemeter, 
the  telephone  being  an  active  part  of  the 
oscillation  circuit.  The  telephone  contains 
a  winding  of  three  or  four  turns  placed 
underneath  a  copper  diaphragm.  The  max- 
imum sound  is  obtained  when  the  wave- 
meter  is  in  resonance  with  a  given  oscilla- 
tion circuit,  the  copper  diaphragm  moving 
with  the  group  frequency  of  the  transmitter. 
In  Fig.  210h,  a  tube  N,  filled  with  neon 
gas,  has  sealed-in  terminals  at  either  end. 
When  shunted  across  the  terminals  of  the 
wavemeter  condenser,  the  tube  glows  bril- 
liantly at  resonance. 

In  Fig.  210i,  a  crystal  rectifier  D  is  connected  in  series  with  a  2,000  ohm  telephone  P,  the 
final  two  terminals  being  shunted  across  the  condenser  C.  The  maximum  of  sound  is  obtained 
in  the  telephone  at  resonance.  The  connection  of  210j  is  often  preferred  because  the  calibra- 
tion of  the  wavemeter  is  not  affected  by  the  presence  of  a  shunt  detecting  circuit  as  in  210i. 
The  uni-polar  connection  of  the  detector  has  the  disadvantage  that  the  wavemeter  must  be 
placed  in  closer  inductive  relation  to  the  circuit  under  test  than  with  the  connection  of  21  Oi. 
The  inspectors  of  the  American  Marconi  Company  prefer  either  the  hot  wire 
wattmeter,  or  the  crystal  detector  and  head  phones  as  indicators  of  resonance 
above  all  others. 

162.  Uses  of  the  Wavemeter. — The   wavemeter  may  be   employed: 

(1)  To  place   two    or   more   circuits   of   radio-frequency   in   resonance; 

(2)  To  measure  the  wave  length  of  the  closed  or  open  oscillation  circuits  of 
a  radio  transmitter. 

(3)  To  determine  the  percentage  of  coupling  between  the  closed  and  open  cir- 
cuits of  a  transmitter; 

(4)  To  determine  the  decrement  of  damping; 

(5)  To  calibrate  a  receiving  set  (by  means  of  an  exciting  buzzer) ; 

(6)  To   determine  the  wave  length  of  a  distant  transmitting  station  at  the 
receiving  station. 

(7)  To  determine  the  purity  of  the  wave  emitted  from  the  antenna. 

163.  Simple  Use  of  the  Wavemeter. — A  simple  use  of  the  wavemeter  is 
shown  in  the  diagram,  Fig.  211,  wherein  the  natural  wave  length  of  an  aerial  is 
to  be  measured.    In  the  diagram,  Fig.  211,  S  is  the  secondary  winding  of  an  induc- 


Fig.  211 — Diagram  for  Measuring  the  Natural  Wave 
Length   of  an  Aerial. 


PRACTICAL  RADIO  MEASUREMENTS. 


191 


tion  coil  fitted  with  a  vibrator,  S-l  a  spark  discharge  gap  of  simple  design  con- 
nected in  series  with  the  aerial  A-l.  L  is  the  inductance  coil  of  the  wavemeter;  C 
the  variable  condenser;  D  a  crystal  rectifier,  and  P  the  head  telephone. 

When  the  induction  coil  is  set  in  opera- 
tion a  spark  discharge  takes  place  at  S-l, 
the  aerial  circuit  being  traversed  by  groups 
of  oscillations  at  a  frequency  determined  by 
the  distributed  values  of  inductance  and 
capacity  of  the  aerial,  or, 
1 


Fig.  212 — Showing  How  an  Aerial  May  Be  Set  Into 
Excitation  by  Buzzer. 


and  a  corresponding  wave  motion  is  prop- 
agated through  space. 

Now  if  the  coil  of  the  wavemeter  L 
bears  the  correct  inductive  relation  to  the 
aerial  wire,  preferably  placed  near  to  the 
earth  lead,  oscillations  of  maximum  ampli- 
tude will  be  induced  in  the  wavemeter  cir- 
cuits when  the  wavemeter  and  the  aerial 
circuit  are  in  exact  resonance.  With  the 
spark  discharging  at  S-l  the  capacity  of  the 


wavemeter  condenser  is  varied  until  a  clear  distinct  sound  is  heard  in  the  head  telephones  at 
a  definite  setting  (of  the  condenser).  The  wave  length  tabulated  on  the  scale  underneath 
the  pointer  is  that  of  the  circuit  under  test. 

Generally  the  resonant  adjustment  of  the  wavemeter  is  sharply  defined,  so  that  a  slight 
change  in  capacity  will  cause  the  signals  to 
disappear.  During  this  measurement,  the 
operator  should  take  care  to  distinguish 
between  the  currents  induced  in  the  wave- 
meter  by  the  oscillations  flowing  in  the 
antenna  circuit  and  those  set  up  by  electro- 
static induction  from  the  induction  coil. 
The  sounds  in  the  telephone  due  to  the 
radio-frequent  oscillations  are  heard  at  a 
distinct  setting  of  the  wavemeter  con- 
denser, but  the  sounds  caused  by  simple 
transformer  induction  can  usually  be  heard 
over  the  entire  condenser  scale.  More  ac- 
curate readings  can  be  obtained  by  placing 
the  coil  of  the  wavemeter  close  to  the  earth 
lead  of  the  transmitter  although  when 
measuring  the  fundamental  wave  length  of 
the  usual  ships  aerial,  by  use  of  a  sensitive 
crystal  rectifier,  the  wavemeter  can  be 
placed  200  or  300  feet  from  the  aerial,  pro- 
vided the  inductance  coil  bears  the  correct 
inductive  relation  to  the  antenna. 

164.  General  Instructions  for 
Tuning  a  Radio-Transmitter. — In 
order  to  tune  a  Badio-transmitter  to  the 
international  standard  wave  lengths, 
the  following  measurements  must  be 
taken : 


I 


Fig.   213 — Another  Method  for  exciting  Oscillations 
in    an    Antenna    by    Buzzer. 


(1)  The  natural  or  fundamental  wave  length   of  the  aerial; 

(2)  The  wave  length  of  the  closed  oscillation  circuit; 

(3)  The  wave  length  of  the  radiated  wave. 

After  the  open  and  closed  circuits  have  been  coupled,  we  must  determine : 


192 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Fig.     214 — Buzzer     Excitation     Circuit     Inductively 
Coupled  to  an  Aerial. 


(1)  Purity  and  sharpness  of  the  radiaied  wave; 

(2)  The  decrement  of  damping. 

(a)  Measurement  of  the  Open  Circuit.  One  method  of  setting  the  aerial  into 
excitation  and  measuring  the  natural  or  fundamental  wave  length  was  shown  in 
Fig.  211,  but  there  are  several  alternative  methods  by  which  the  antenna  may  be 
set  into  oscillation. 

In  the  method  shown  in  the  diagram,  Fig.  212,  a  condenser  C  of  one  microfarad  capacity 

is  connected  in  series  with  the  aerial  and 
also  across  the  vibrator  of  a  buzzer  B-l 
energized  by  the  battery  B-2.  The  counter 
E.  M.  F.  of  the  buzzer  windings  places  an 
electrostatic  charge  on  the  aerial  which  dis- 
charges at  practically  its  natural  frequency. 
The  capacity  of  C  being  very  large  compared 
to  that  of  the  aerial,  the  wave  length  of  the 
system  is  not  appreciably  altered. 

With  the  buzzer  adjusted  for  clear  tones, 
the  wavemeter,  fitted  with  a  crystal  rectifier, 
is  placed  in  inductive  relation  to  the  earth 
lead  and  the  capacity  of  the  wavemeter  con- 

T(C©)  ((©)  denser  varied  until  a  maximum  of  sound  is 

X    '  \Z±y  obtained   in   the   telephone. 

In  the  diagram,  Fig.  213,  a  few  turns  of 
wire  are  inserted  at  L-l  through  which  the 
circuit  of  the  battery  and  buzzer  is  com- 
pleted. When  the  current  flowing  from  B-2 
is  interrupted  by  the  buzzer,  a  change  in  the 
strength  of  the  magnetic  field  takes  place  about  L-l,  placing  a  charge  on  the  aerial  system 
which  discharges  at  its  natural  frequency. 

A  somewhat  similar  arrangement  is  shown  in  Fig.  214,  wherein  the  buzzer  circuit  is  in- 
ductively coupled  to  the  aerial  circuit  through  the  transformer  P,  S,  the  winding  S,  consisting 
of  a  few  turns  of  wire.  The  wavemeter  is  placed  in  inductive  relation  to  some  part  of  the 

open  circuit  and  the  wave 

CLOSED    CIRCUIT  length  noted  by  adjusting 

the  wavemeter  to  reson- 
ance. 

The  aerial  frequently 
is  set  into  excitation  by 
connecting  the  antenna 
and  earth  wire  of  an 
aerial  system  across  the 
vibrator  of  a  buzzer,  but 
this  method  of  excitation 
is  not  as  satisfactory  as 
the  connections  of  the 
two  methods  shown  pre- 
viously. 

To  tune  the  aerial  in 
either  of  the  foregoing 
diagrams  to  the  standard 
wave  of  600  meters,  turns 
of  inductance  are  added 
at  winding  L-2  until  the 
wavemeter  indicates  the 


Fig.   215 — Showing  How  the  Wave  Length  of  a  Closed  Oscillation   Circuit 
is    Determined. 


wave  length  to  be  600 
meters.  Generally  from 
10  to  20  microhenries  of  inductance  are  included  at  L-l  to  constitute  the  secondary  winding 
of  the  oscillation  transformer,  the  additional  turns  required  for  any  standard  wave  being 
added  in  the  aerial  tuning  inductance. 

The  aerial  is  tuned  to  the  300  meter  wave  by  connecting  the  short  wave  condenser  in 
series  with  the  aerial  system.    A  reading  is  then  taken  on.  the  wavemeter,  to  note  the  reduc/- 


PRACTICAL  RADIO  MEASUREMENTS. 


193 


V 

m 


tion  of  wave  length.  If  the  length  of  the  wave  exceeds  300  meters,  turns  are  taken  out  at 
L-2  until  the  standard  wave  is  obtained.  Certain  aerials  encountered  in  practical  radio  have 
excessive  length  for  radiating  the  300  meter  wave  even  with  the  aerial  tuning  inductance  cut 
out.  In  a  case  of  this  kind,  the  aerial  must  be  reduced  in  length  or  the  lead-ins  attached  to 
the  center  of  the  flat  top  rather  than  to  one  end. 

The  correct  number  of  turns  for  the  450  meter  wave  are  found  in  the  same 
manner  as  for  the  300  and  600  meter  waves.  Generally  the  short  wave  condenser 
is  cut  out  of  the  circuit  at  this  adjustment. 

(b)  Measurement  0/  the  Closed  Circuit.    The  closed  circuit  is  tuned  to  the 
standard  waves  by  placing  the  wavemeter  in  inductive  relation  to  the  primary 
winding  of  the  os- 
cillation transform- 
er, care  being  taken 

to  disconnect  the 
open  circuit  from 
the  secondary 
winding.  The  spark 

is  then  discharged  — . 

in  the  regular  man-  lilihl      f 

ner  and  the   wave  '  V'  / 

length  for  a  given 
value  of  primary 
inductance  found 
on  the  wavemeter 
by  means  of  any  of 
the  resonance  indi- 
cators described  in 
paragraph  161. 

A  diagram  of  con- 
nections for  this 
measurement  is  shown 
in  Fig.  215.  Contact 
"T"  is  connected  in  at 
various  points  on  the 
coil  until  the  re- 
quired wave  is  found. 
With  transmitting 
sets  of  large-r  rating 
the  capacity  of  the 
closed  circuit  con- 
denser may  have  to 
be  reduced  to  obtain 
the  300  meter  wave. 

(c)  Measure- 
ment of  the  Radi- 
ated   Wave.      The 


WATTMETER 
Fig.   216 — Showing   Method  of  Determining   Length   of   the   Radiated   Wave. 


next  step  in  tuning  is  to  ascertain  if  the  emitted  wave  is  pure  and  sharp  in  ac- 
cordance with  the  U.  S.  statute  requirements.  To  determine  this,  the  closed  and 
open  oscillation  circuits  are  tuned,  inductively  coupled  and  set  at  the  coupling, 
giving  the  highest  ammeter  reading.  (Fig.  216.) 

Next  a  test  is  made  to  determine  if  the  aerial  radiates  all  its  energy  at  one  wave  length. 
If  the  antenna  oscillates  at  two  frequencies,  two  resonant  adjustments  will  be  obtained  on 
the  wavemeter  and  the  relative  amplitude  of  the  two  frequencies  can  be  measured  by  a  zvatt- 
meier  "W"  connected  in  the  circuit  of  the  wavemeter.  With  the  wavemeter  in  a  fixed  posi- 
tion relative  to  the  antenna,  the  meter  is  adjusted  to  resonance  and  the  relative  power  at  the 
two  frequencies  of  oscillation  observed.  Unless  this  measurement  is  taken  with  caution,  the 
wattmeter  may  burn  out  if  the  wavemeter  is  placed  too  close  to  the  circuits  of  radio- 
frequency. 


194  PRACTICAL   WIRELESS   TELEGRAPHY. 

According  to  the  U.  S.  regulations,  the  radiated  wave  is  considered  as  being  pure,  ivhen 
(if  the  antenna  oscillates  at  tzvo  frequencies}  the  amplitude  of  the  energy  corresponding  to 
the  smaller  wave  is  10  per  cent,  or  less  than  that  in  the  greater  wave. 

To  determine  the  relative  power  of  the  radiated  waves,  the  capacity  of  the  wavemeter 
condenser  must  be  carefully  varied,  the  two  maximum  readings  of  the  wattmeter  correspond- 
ing to  the  two  oscillation  frequencies  being  particularly  noted.  If  the  wattmeter  indicates 
0.1  watt  at  the  longer  wave,  and  0.01  watt  or  less  at  the  shorter  wave,  the  wave  is  pure  ac- 
cording to  the  United  States  regulations.  If  the  amplitude  of  the  shorter  wave  exceeds  0.01 
watt,  the  coupling  between  the  primary  and  secondary  windings  must  be  reduced  until  the 
correct  current  amplitudes  are  obtained  or  until  a  single  wave  emission  results.  It  is  usual 
to  reduce  the  coupling  until  the  antenna  radiates  a  single  wave. 

That  two  waves  are  being  radiated  from  an  aerial  can  be  determined  by  a  crystal  rectifier 
and  telephone  as  well  as  by  the  wattmeter,  but  the  relative  power  of  the  waves  cannot  be 
determined  in  this  way.  If  a  sensitive  vacuum  valve  detector  is  connected  to  the  wavemeter, 
the  latter  can  be  placed  several  hundred  feet  from  the  aerial.  The  wave  length  can  be 
measured  with  a  greater  degree  of  accuracy,  due  to  the  "looseness"  of  coupling. 

When  a  ship's  aerial  is  tuned  to  the  standard  waves,  near  to  a  building  with  a 
steel  or  iron  frame,  the  effective  resistance  and  the  natural  wave  length  are  ap- 
preciably altered;  hence,  to  secure  the  maximum  flow  of  antenna  current,  the 
aerial  tuning  inductance  must  be  slightly  readjusted  (when  the  ship  is  at  sea) 
until  the  ammeter  indicates  a  maximum.  Similar  effects  are  observed  when  a  ship 
lies  near  to  another  vessel  equipped  with  an  aerial,  particularly  if  the  aerial  of  the 
other  vessel  is  connected  to  earth.  It  is  preferable  to  tune  a  transmitting  set  at 
a  distance  of  at  least  a  half  wave  length  from  large  metallic  structures. 

We  have  defined  a  pure  wave  according  to  the  U.  S.  regulations  as  being  one 
in  which,  if  the  antenna  oscillates  at  two  frequencies,  the  energy  of  the  lesser 
wave  will  not  exceed  by  10  per  cent,  the  energy  in  the  greater  wave.  According 
to* the  same  regulations,  a  sharp  wave  is  one  in  which  the  decrement  of  damping 
per  complete  oscillation  is  0.2  or  less.  The  measurement  for  the  decrement  will 
be  described  in  paragraph  169. 

165.  Tuning  by  the  Hot  Wire  Ammeter. — The  open  and  closed  circuits 
of  a  transmitting  set  can  be  tuned  to  resonance  as  follows :    Set  the  closed  oscillation  circuit 
to  a  de-Unite  wave,  or  to  one  of  the  three  standard  waves,  by  means  of  a  wavemeter.    Place 
the  secondary  winding  of  the  oscillation  transformer  in  inductive  relation  to   the  primary 
winding.     Follow  it  by  cutting  in  and  out  turns  at  the  aerial  tuning  inductance  until  the 
aerial  ammeter  indicates  a  maximum  of  current. 

Next  change  the  coupling  and  leave  the  primary  and  secondary  coils  at  that  position, 
where  a  fair  value  of  antenna  current  is  obtained,  with  the  wave  emission  complying  with 
the  law  in  respect  to  sharpness  and  purity.  (The  latter  is  determined  by  the  decremeter.) 

The  method  can  be  reversed  as  follows :  Set  the  open  circuit  to  a  standard  wave  by  the 
wavemeter  and  place  the  primary  winding  of  the  transmitting  oscillation  transformer  in 
inductive  relation  to  the  antenna  coil.  With  the  spark  discharging  at  normal  power  con- 
sumption, add  or  subtract  primary  turns  until  the  highest  possible  reading  of  the  aerial 
ammeter  is  obtained. 

A  small  glow  lamp,  such  as  a  2,  4  or  6  volt  battery  lamp,  shunted  by  a  semi-loop  of  wire, 
serves  as  an  indicator  of  resonance  between  the  open  and  closed  circuits  and  is  often  used 
in  case  of  emergency. 

166.  Tuning   the   2   K.   W.   500   Cycle   Panel   Transmitter.— Detailed   in- 
structions for  the  tuning  of  the  standard  panel  sets  of  the  Marconi  Company  will 
be  given  in  Part  XII.    The  process  is  quite  similar  to  the  tuning  of  any  set,  with 
the  exception  that  the  closed  oscillation  circuit  is  tuned  to  the  three  standard 
waves  at  the  Marconi  factory  and  outside  of  the  determination  of  the  logarithmic 
decrement  all  tuning  aboard  ship  can  be  done  by  the  aerial  ammeter  alone. 

The  particular  feature  of  this  set  is  the  wave  length  changing  switch,  which  in  a  simple 
operation  permits  either  the  300,  450  'or  600  meter  waves  to  be  radiated,  the  necessary  changes 
of  self -inductance,  condenser  capacity,  and  coupling  being  selected  by  a  set  of  blades  making 
contact  with  studs  leading  to  various  parts  of  the  circuit.  The  method  of  tuning  this  set 
will  be  clear  if  the  following  facts  are  kept  in  mind : 


PRACTICAL  RADIO  MEASUREMENTS.  195 

(1)  For  any  particular  wave  length  adjustment  of  the  transmitter,  a  critical 

degree   of  coupling   between   the  primary  and  secondary  windings   gives 

the  maximum  aerial  current. 
*(2)  The  coupling  for  the  quenched  spark  discharger  can  be  closer  than  for  the 

rotary  discharger,  if  the  quenched  gap  is  in  perfect  condition. 
(3)  The  coupling  between  the  primary  and  secondary  circuits  of  an  oscillation 

transformer  can  be  varied  by  increase  or  decrease  of  the  self-induction  of 

either  coil  as  well  as  by  drawing  the  coils  apart. 

When  the  quenched  discharger  is  in  use,  the  primary  winding  bears  a  fixed  mechanical  posi- 
tion to  the  secondary  winding,  but  the  oscillation  transformer  is  constructed  so  that  they 
can  be  drawn  apart  for  use  of  the  rotary  gap.  For  the  quenched  gap,  the  requisite  coupling 
for  each  standard  wave  is  secured,  by  variation  of  the  secondary  inductance,  turns  being 
added  at  the  aerial  tuning  inductance  to  maintain  resonance. 

The  process  of  tuning  these  sets  not  only  consists  of  adjusting  the  open  and  closed  cir- 
cuits to  resonance,  but  also  in  locating  the  proper  secondary  inductance  for  the  maximum 
aerial  current.  It  is  more  or  less  a  cut  and  try  method,  but  the  skill  obtained  through 
practice  enables  an  inspector  to  complete  the  tuning  in  half  an  hour.  The  exact  location  of 
the  secondary  contact  clips  can  be  found  more  rapidly  by  turning  the  coupling  handle  on  the 
front  of  the  panel  set,  and  thereby  mechanically  placing  the  primary  winding  closer  to  the 
secondary  or  farther  away.  If  the  primary,  for  instance,  is  drawn  away  from  the  secondary 
and  the  aerial  ammeter  registers  an  increase  of  current,  it  is  an  indication  that  the  coupling 
for  the  final  -fixed  position  of  the  windings  is  too  close.  Hence  the  primary  winding  is 
brought  back  to  its  original  position  and  turns  taken  out  at  the  secondary,  followed  by 
adding  turns  at  either  of  the  aerial  tuning  inductances  until  resonance  and  maximum  read- 
ing of  the  ammeter  is  secured. 

The  set  should  be  tuned  for  the  three  standard  waves,  with  approximately  three  turns 
cut  in  at  the  continuously  variable  aerial  tuning  inductance.  After  the  position  of  the  taps 
for  the  secondary  winding  and  the  plug  aerial  tuning  inductance  are  once  located,  no  further 
adjustment  is  required,  unless  some  change  in  the  antenna  is  made.  With  these  sets  the 
complete  tuning  process  can  be  carried  out  by  observation  of  the  aerial  ammeter  alone,  but 
the  wavemeter  should  be  employed  to  measure  the  decrement  and  to  note  if  two  waves  are 
being  radiated. 

The  tuning  of  the  ^  k.  w.  set  is  similar  to  that  of  the  2  k.  w.  set. 

In  the  tuning  of  a  quenched  spark  transmitter,  two  or  even  three  points  of  coupling  for 
maximum  reading  of  the  aerial  ammeter  are  frequently  obtained  for  a  given  wave  length  ; 
the  coupling  to  be  finally  selected  is  that  giving  the  maximum  value  of  aerial  current.  When 
the  rotary  gaps  of  these  sets  are  in  use,  the  primary  and  secondary  windings  are  separated 
by  a  space  of  8  to  13  inches,  but  the  coupling  position  may  have  to  be  altered  slightly  for 
each  of  the  three  standard  waves. 

167.  Determination     of     Coupling.  —  It     is     customary,     in     transmitting 

systems  employing  the  plain  spark  discharger,  to  express  the  degree  of  coupling  between 
the  open  and  closed  oscillation  circuits  in  terms  of  a  percentage  rather  than  in  terms  of  the 
true  co-efficient  of  coupling.  The  percentage  of  coupling  is  obtained  by  placing  the  wave- 
meter  in  inductive  relation  to  the  aerial  and  observing  the  length  of  the  radiated  waves.  If 
the  readings  of  the  longer  and  shorter  waves  are  squared  and  their  values  inserted  in  the 
following  formula,  the  coupling  is  obtained,  or  : 

X2-  —  Xis 
K  =  -  X  100 

\22  -f-  Xl2 

Where  \  =  longer  wave, 
X~  —  shorter  wave. 

The  true  coefficient  of  coupling  is  obtained  from  the  following  formula  : 

M 


Where  M  ^mutual  inductance  of  the  oscillation  transformer, 
L±  =self-indUctance  primary, 
L2  =self-inductance  secondary. 

*Statements    (1),    (2)   and   (3)    apply  to  all   coupled  transmitters  of  the   American   Marconi  Company. 


196 


PRACTICAL  WIRELESS  TELEGRAPHY. 


168.  Plotting  of  Resonance  Curves. — A  calibration  of  the  closed  and 
open  circuits  of  a  radio-transmitter  is  frequently  desired  to  determine,  let  us  say, 
the  variation  of  frequency  or  wave  length  occasioned  by  the  insertion  of  a  certain 
amount  of  inductance.  The  results  of  such  measurements  are  plotted  on  cross- 
section  paper  in  the  form  of  a  curve  (or  perhaps  a  straight  line).  The  principal 
advantage  of  such  plotting  lies  in  the  fact  that  it  enables  one  to  take  a  character- 
istic set  of  readings  at  two  or  more  decisive  points  and  thus  determine  interme- 
diate values  by  following  the  general  outline  of  the  curve. 


AERIAL  TUNING 
INDUCTANCE 


WATTMETER, 


Fig.   217 — Complete  Apparatus   for  Measuring  Decrement  of  Antenna   Oscilla- 
tions   and    Plotting    a    Resonance    Curve. 

The  resonance  curve  of  the  radiated  wave  of  a  transmitter  is  a  graphic  way  of  showing 
the  relation  between  the  amplitude  of  the  current  and  the  frequency  or  wave-lengths  at  and 
near  the  fundamental  frequency.  The  value  of  the  plotting  lies  in  the  fact  that  it  enables 
the  experimenter  to  obtain,  in  a  general  way,  the  over-all  distribution  of  energy  in  the  emitted 
wave,  the  decrement  of  damping  and  the  relative  power  of  the  two  waves,  if  present. 

To  plot  the  resonance  curve  of  the  radiated  wave,  a  wavemeter  is  required,  in  series 
with  which  is  connected  either  a  hot  wire  milliammeter  or  a  hot  wire  wattmeter.  The 
milliammeter  should  have  a  range  of,  say  40  to  240  milliamperes,  while  the  wattmeter  may 
have  a  scale  reading  of  .01  to  .1  watt.  The  latter  instrument  is  preferably  connected  to  the 
secondary  winding  of  a  step-down  transformer,  the  primary  winding  of  which  is  connected 


PRACTICAL  RADIO  MEASUREMENTS. 


197 


in  series  with  the  circuit  of  the  wavemeter.  This  transformer,  as  a  rule,  is  made  up  of 
Litzendraht  wire,  the  primary  consisting  of  ten  turns  wound  on  a  wooden  spool,  about  \l/2 
inches  in  length  by  1  inches  in  diameter.  The  secondary  winding  consists  of  five  turns  of 
the  same  wire  wound  directly  over  it  with  a  single  layer  of  empire  cloth  between. 

The  wattmeter  having  been  connected  in  the  circuit  of  the  wavemeter,  a  few  preliminary 
trials  are  made  to  determine  the  proper  inductive  relation  between  the  coil  of  the  wavemeter 
and  the  antenna  circuit.  Care  must  be  taken  that  the  wattmeter  is  not  burned  out  at  the 
point  of  resonance.  The  proper  inductive  relation  having  been  obtained  by  trial,  the  coil 
of  the  wavemeter  is  placed  in  such  position  as  to  give  a  near  maximum  scale  deflection  of 
the  wattmeter  at  the  resonant  adjustment  for  the  longer  wave.  If  maximum  deflection  of 
the  wattmeter  is  obtained  at  resonance,  it  is  evident  that  the  current  will  decrease  at  wave- 
lengths off  resonance,  the  reduction  being  dependent  upon  the  decrement  of  the  oscillations. 

Readings  of  the  indicating  instrument  having  been  observed  at  resonance,  another  read- 
ing is  taken  at  a  point  off  resonance  where  the  wattmeter  shows  a  small  scale  deflection,  say, 
.01  watt.  Similar  observations  are  made  at  other  wave  lengths  approaching  resonance  and 
beyond  resonance  until  a  full  set  of  calibrations  are  secured;  that  is  to  say,  the  observations 
are  continued  to  a  point  beyond  resonance  where  a  nearly  zero  deflection  of  the  wattmeter 
is  obtained. 

A  diagram  of  connections  and  the  relative  positions  of  the  apparatus  for  this  de- 
termination are  shown  in  Fig.  217,  where  the  closed  oscillatory  circuit  of  a  radio  transmitter 
is  represented  by  the 
primary  winding,  the 
high  potential  condenser 
and  the  rotary  spark  gap. 
The  antenna  system  in- 
cludes the  secondary 
winding,  the  aerial  tun- 
ing inductance  and  the 
earth  wire,  which  pref- 
erably has  a  single  turn 
of  wire,  L-l,  inserted  in 
series.  The  coil  of  the 
wavemeter,  L,  is  placed 
in  inductive  relation  to 
L-l  and  is  in  series  with 
the  primary  winding  of 
the  step-down  transform- 
er, T,  and  the  variable 
condenser,  C.  The  sec- 
ondary of  this  trans- 
former is  connected  to  a 
low-range  radio-fre- 
quency wattmeter. 

Assume,  for  example, 
that  the  spark  is  dis- 
charging and  that  the 
closed  and  open  oscilla- 
tion circuits  are  in  exact 
resonance ;  furthermore, 
that  two  positions  on  the 
variable  condenser  which 
we  may  call  C-l  and  C-2  are  the  points  at  which  the  wavemeter  is  in  resonance  with  the 
double  wave.  Then,  as  the  pointer  of  the  variable  condenser  is  moved  from  zero  position 
toward  C-l,  the  reading  of  the  wattmeter  increases,  until  the  point,  C-l,  is  passed,  when  a 
decrease  takes  place.  An  increase  of  current  again  takes  place  as  C-2  is  approached  fol- 
lowed by  a  decrease  when  the  point  of  resonance  is  passed.  If  the  wave-length  of  the  wave- 
meter  and  the  corresponding  deflection  of  the  hot  wire  wattmeter  be  observed  over  a  series 
of  wave-lengths,  the  data  thus  obtained  may  be  plotted  in  the  form  of  a  resonance  curve  in 
the  following  manner.  .  (See  Fig.  218). 

Placing  in  one  column  the  wave-lengths  corresponding  to  the  condenser  scale  of  the  wave- 
meter,  and  in  the  second  column  the  corresponding  deflection  of  the  hot  wire  wattmeter, 


500 


800 


600  700 

WAVE  LENGTH 
Fig.    218 — Resonance    Curve    of    the   Antenna    Oscillations. 


900 


198 


PRACTICAL   WIRELESS   TELEGRAPHY. 


co-ordinate  points  are  laid  off  on  cross-section  paper  through  which  a  common  line  or  curve 
is  drawn.     A  typical  set  of  readings  follows: 


Wave-length  of  the 
wavemeter 

450 

495 

525 

535 

540 

560 

575 

585 

600 

615 

625 

635 

655 

700.. 


Corresponding  deflection 
of  the  hot  wire  wattmeter 

0.0 

0.005 

0.01 

0.009 

0.01 

0.03 

0.05 

0.08 

0.1 

0.085 

0.05 

0.03 

0.01 

..0.00 


With  the  cross-section  paper  before  us,  the  wave-length  readings  are  laid  off  horizontally 
as  indicated  in  Fig.  218  and  are  known  as  the  abscissas  of  the  points  on  the  curve,  while  the 
hot  wire  wattmeter  readings  are  laid  off  vertically  and  are  known  as  the  ordinates  of  the 
points  on  the  curve.  Take,  for  example,  the  wave-length  of  575  meters ;  the  corresponding 


300 


400 


500  600  700 

WAVE  LENGTH    IN  METERS 


800 


Fig. 


219— Curves   Showing  the   Change   in  Wave   Length  by   Addition  of  Inductance   in   a   Radio-Frequency 

Circuit. 


PRACTICAL  RADIO  MEASUREMENTS.  199 

hot  wire  wattmeter  deflection  is  .05.  Then  follow  the  vertical  line  corresponding  to  575 
until  the  horizontal  line  is  met  corresponding  to  .05  and  place  a  dot  or  a  cross.  Proceed 
similarly  with  the  entire  set  of  calibrations  until  all  points  on  the  curve  are  located.  Then 
draw  a  line  joining  them. 

Now,  if  the  coil,  L,  of  the  wavemeter  remains  in  the  same  position  relative  to  the  coil,  L-l, 
and  the  primary  and  secondary  windings  of  the  oscillation  transformer,  are  set  at  various 
couplings,  the  curve  in  Fig.  218  will  become  "sharper"  or  "broader,"  accordingly,  as  the 
coupling  is  decreased  or  increased.  By  means  of  these  curves  the  relative  sharpness  of  the 
radiated  waves  can  be  compared  allowing  to  some  extent  a  predetermination  of  the  amount 
of  interference  to  be  expected.  If  the  coupling  at  the  oscillation  transformer  is  properly  re- 
duced, the  lower  peak  of  the  radiated  wave,  which  in  the  curve  exists  at  about  520  meters, 
may  completely  disappear  and  a  single  sharp  maximum  peak  result.  The  resonance  curve 
shown  in  Fig.  218  is  not  that  of  a  "sharp"  wave  in  compliance  with  the  United  States  restric- 
tions, unless  the  decrement  of  the  antenna  current  is  0.2  or  less  per  complete  oscillation.  It 
is,  however,  "pure"  as  far  as  the  relative  amplitudes  of  the  two  waves  are  concerned. 

Should  the  wavemeter  indicate  no  sharp  or  defined  point  of  resonance,  it  may  be  due  to  an 
excess  of  coupling  at  the  oscillation  transformer,  to  high  resistance  joints  or  to  poor  con- 
nections in  the  antenna  circuit.  Again,  this  may  lie  due  to  leakage  across  the  antenna 
insulators;  the  remedy,  of  course,  is  obvious. 

Using  the  apparatus  employed  to  obtain  data  for  a  resonance  curve,  the  student  may 
obtain  data  and  construct  other  curves  indicating  the  wave  length  of  the  open  and  closed 
oscillatory  circuits  with  various  turns  of  inductance  cut  in  at  the  primary  and  secondary 
windings.  For  example,  if  the  increase  in  wave  length  brought  about  by  adding  turns  in  the 
closed  circuit  is  to  be  determined,  the  wavemeter  is  placed  in  close  inductive  relation  to  the 
primary  inductance,  but  not  too  close  to  burn  out  the  wattmeter  I.  Starting  with  one  turn 
in  the  closed  circuit,  the  wave  length  is  determined  by  observing  the  wavemeter  reading  corre- 
sponding to  maximum  deflection  of  the  wattmeter  (with  spark,  of  course,  discharging). 
Turns  are  then  added  progressively  at  the  secondary  (from  one  to  maximum)  until  a  com- 
plete set  of  calibrations  are  obtained.  The  results  of  a  typical  set  of  calibrations  follow : 

Corresponding  wave- 
Turns  lengths  in  meters 

1 404 

2 416 

3 428 

4 444 

5 458 

6 472 

7 482 

8 488 

9 495 

10 505 

11 514 

12 528 

Plotted  in  curve  form  these  data  appear  in  curve  A,  Fig.  219.  Here  the  wave-lengths  (in 
meters)  are  the  abscissas  of  the  points  on  the  curve  and  the  helix'  turns  the  ordinates  of  the 
points  on  the  curve.  The  wave  lengths  corresponding  to  fractions  of  a  turn  on  the  helix 
can  be  determined  by  following  the  abscissae  to  the  base  line  or  the  horizontal  axis. 

We  can  plot  the  calibrations  of  the  open  circuit  of  a  transmitting  system  in  a  similar  way, 
but  in  this  case  a  spark  gap  must  be  connected  in  series  with  the  antenna  system  and  in  turn 
connected  to  the  secondary  winding  of  an  induction  coil.  Generally  the  oscillations  flowing 
in  the  antenna  circuit  are  not  powerful  enough  with  this  connection  to  operate  the  wattmeter 
(in  series  with  the  wavemeter),  or  the  spark  discharge  of  the  coil  may  be  too  irregular  to 
permit  a  reading  of  the  wattmeter  to  be  observed.  It  is  therefore  preferable  to  shunt  the 
wavemeter  by  a  crystalline  detector  and  head  telephone,  the  point  of  resonance  being  located 
by  the  maximum  sound  in  the  receiver. 

Turns  are  added  in  the  antenna  circuit  one  at  a  time  and  corresponding  wave  length 
readings  observed  on  the  wavemeter.  A  typical  set  of  readings  follows : 


200  PRACTICAL  WIRELESS  TELEGRAPHY. 

Corresponding  wave- 
Turns  lengths  in  meters 

1 290 

2 410 

3 525 

4 600 

5 683 

6 770 

These  data  appear  in  the  curve,  B,  Fig.  219.  Intermediate  wave  lengths  lying  between  the 
complete  turns  may  be  found  as  in  the  previous  case. 

Curves  of  the  latter  type  should  be  prepared  at  each  commercial  station  and  posted  in  the 
apparatus  room  for  quick  reference.  In  this  manner,  the  open  and  closed  oscillatory  circuits 
may  be  tuned  to  the  standard  waves  with  the  assurance  that  the  circuits  are  in  exact 
resonance. 

Returning  to  the  resonance  curve  of  Fig.  218 :  Assume  that  the  portion  of  the  curve,  Fig. 
218,  above  the  horizontal  line  marked  .05  represents  the  range  of  wave  lengths  over  which 
the  signals  are  audible  at  a  receiving  station  located,  say,  75  miles  distant  from  the 
transmitter. 

It  is  easily  seen  that  a  change  of  20  meters  on  either  side  of  the  maximum  ordinate  will 
reduce  the  incoming  signals  to  zero.  It  is  also  plain  that  the  sharper  the  curve,  the  greater 
will  be  the  reduction  of  the  strength  of  the  incoming  signal  for  a  given  amount  of  detuning 
at  the  receiver.  Hence  resonance  curves,  in  a  sense,  enable  us  to  determine  the  ratio  of  the 
current  at  the  receiver  for  a  given  wave  length  off  resonance  to  that  obtained  at  the  maxi- 
mum ordinate  or  resonance. 

169.  Measurement  of  the  Logarithmic  Decrement  of  Damping. — The  de- 
termination of  the  decrement  of  damping  of  the  antenna  oscillations  is  a  measure 
of  the  tuning  qualities  of  a  radio-transmitter  and,  in  a  sense,  permits  a  prede- 
termination of  the  interference  to  be  expected  therefrom.  If  the  decrement  of  the 
oscillations  flowing  in  the  open  circuit  is  known,  we  may  determine  the  number 
of  oscillations  per  spark  discharge  as  follows : 

4.605  +  8 
M=-  (1) 

8 
where  8  =  Naperian  logarithm  of  the  ratio  of  two  successive  oscillations 

in  the  same  direction ; 

M  =  number  of  complete  oscillations  in  the  train  when  the  last  one 
has  attained  amplitude  of  .01  of  the  initial  oscillation. 

Hence  if  the  emitted  wave  has  a  decrement  of  0.2  there  will  be  approximately  24 
complete  oscillations  per  spark. 

It  has  been  determined  by  experiment  that  a  transmitter  having  decrement  in  excess  of  .2 
per  complete  oscillation  causes  excessive  interference  to  the  operation  of  other  stations  not 
tuned  to  the  same  wave  length,  and  it  necessarily  follows  that  the  greater  the  decrement  of 
the  radiated  wave  the  greater  will  be  the  interference,  e.  g.,  the  wave  will  lack  tuning  quali- 
ties. Hence,  as  far  as  tuning  and  the  elimination  of  interference  is  concerned,  it  is  desirable 
to  keep  the  decrement  at  a  minimum  value. 

Modern  quenched  spark  transmitters  have  oscillation  decrements  lying  between  .05  and  .12 
and  therefore  set  up  a  minimum  of  interference,  to  the  operation  of  other  stations. 

It  has  been  shown  by  V.  Bjerknes  that  the  combined  decrement  of  two  circuits  of  radio- 
frequency,  magnetically  coupled,  is  given  by  the  following  formula: 


(Xi  \   r 
i  --    |V 
Xr     / 


(2) 


I'2 


Where  (in  the  case  where  a  wavemeter  is  coupled  to  the  antenna  system  of  a  wireless  tele- 
graph transmitter), 


*Some  writers  express  the  logarithmic  decrement  on  the  basis  that  the  amplitude  of  the  last  oscillation 
is    one-tenth    of    the    initial    oscillation. 


PRACTICAL  RADIO  MEASUREMENTS. 


201 


5i  —  decrement  of  the  circuit  under  measurement, 
&j  =:  decrement  of  the  decremeter  or  wavemeter, 
\r  =  wave  length  at  resonance, 

Xi  =  wave  length  not  more  than  3  per  cent,  to  5  per  cent,  off  resonance, 
1J  =:  current  indicated  by  a  measuring  instrument  connected  in  series  with  the 

wavemeter  when  the  wavemeter  is  adjusted  to  Xt. 

I rs  —  current  indicated  as  above  when  the  wavemeter  is  adjusted  to  resonance. 
For  accuracy  52  must  be  small  as  compared  with  Si. 

In  terms  of  the  condenser  capacity  of  the  wavemeter,  the  foregoing  formula  can  be  written 
as    follows. 


C'  —  O 


5i  4-  S3  = 


(3) 


Where  Cr  =  capacity  of  the  wavemeter  condenser  at  resonance; 

G  =:  capacity  of  the  wavemeter  condenser  of  a  certain  point  off  resonance. 

It  has  been  found  for  all  practical  purposes,  that  if  the  value  of  I'  is  selected  to  be  one- 
half  the   value  of   I"-",  the   formula  is   sufficiently  accurate;   hence,   the   integers   underneath 


Fig.  220 — The  American  Marconi  Company's  Decremeter. 


the  radical  may  be  cancelled  and  the  combined  and  total  decrement  of  the  two  circuits  obtain 
from  the  following  simple  formula: 


Cr  —  G 
Cr 


(4) 


Now,  it  is  found  by  experiment  that  the  resonance  curve  of  a  radio-transmitter  is  not 
always  symmetrical  and,  therefore,  different  values  for  the  decrement  will  be  found  accord- 
ingly as  the  measurement  is  made  on  either  side  of  the  maximum  ordinate.  Consequently  the 
results  will  be  more  accurate  if  the  capacity  of  the  condenser  is  taken  above  and  below 
resonance  at  points  where  the  corresponding  flow  of  current  as  measured  by  the  wattmeter 
is  one-half  the  value  obtained  at  resonance;  then  the  mean  value  of  the  decrement  will  be 
determined  as  follows : 


202 


PRACTICAL   WIRELESS   TELEGRAPHY. 
—  Ci   TT 


-f  «•  — - 


(5) 


where  Cs  —  the  capacity  of  the  condenser  at  a  value  greater  than  C«-,  the  capacity 
where  the  reading  of  the  wattmeter  falls  to  one-half  that 
obtained  at  Cr  and, 

Q  =  the  capacity  of  the  condenser  at  a  similar  wattmeter  reading  corre- 
sponding to  a  capacity  less  than  Cr. 

Although  the  logarithmic  decrement  of  a  transmitter  may  he  measured  by  several  methods, 
but  one  considered  the  most  practical  by  U.  S.  engineers  will  be  described. 

To  carry  out  this  measurement,  a  wavcmcter  fitted  with  a  current  indicating  instrument 
is  required,  which  may  be  either  a  hot  wire  milliammeter  or  a  hot  wire  wattmeter.  The  con- 
denser of  the  wavemeter  should  have  a  chart  showing  the  capacity  in  microfarads  corre- 
sponding to  each  position  of  the  pointer  on  the  condenser  scale  and  the  hot  wire  wattmeter 
should  have  a  range  of  .01  to  .1  watts. 

To  measure  the  decrement,  the  wattmeter  should  be  connected  as  in  Fig.  217  for  obtaining 
the  data  for  plotting  a  resonance  curve. 

The  coil,  L,  of  the  wavemeter  is  then  placed  in  inductive  relation  to  the  antenna  system 


Fig.    221 — Kolster   Decremeter — the  Type   Used  by   the   Government   Radio   Inspectors. 

at  the  single  turn  of  wire,  L-l.  The  key  of  the  transmitting  apparatus  is  held  down  con- 
tinuously and  the  length  of  the  spark  gap  adjusted  until  the  note  is  clear  and  uniform.  This 
is  followed  by  variation  of  capacity  of  the  wavemeter  condenser  until  the  wattmeter  indictes 
a  maximum  deflection  which,  of  course,  is  obtained  at  the  point  of  resonance 
between  the  two  circuits.  The  inductance  coil  L  is  then  moved  about — that  is  to  say — the 
coupling  between  it  and  the  antenna  coil  L-l  is  changed  until  the  maximum  reading  of  the 
wattmeter  falls  (as  a  matter  of  convenience)  on  some  even  number,  say  0.08  watt.  The 
condenser  is  then  set  at  a  lower  value  of  capacity,  O,  where  the  reading  of  the 
wattmeter  is  0.04  watt.  A  value  of  capacity  above  Cr  is  then  observed  (Cs)  where  the 


PRACTICAL  RADIO  MEASUREMENTS. 


203 


reading  of  the  wattmeter  again  falls  to  0.04  watt.  Substituting  these  values  of  capacity  in 
formula  No.  4  the  combined  decrement  of  the  two  circuits  is  obtained  by  a  simple  calculation. 
Now,  if  the  decrement  of  the  wavemeter  is  subtracted  from  the  total  value  obtained  by 
the  last  equation,  the  result  will  be  the  decrement  of  the  radiated  wave.  The  decrement  of 
the  wavemeter  can  be  obtained  by  direct  calculation  or  by  actual  measurement. 

170.  Calculation  of  the  Decrement  of  the  Wavemeter  (or  Decremeter). — 
One  method  by  which  the  damping'  of  the  wavemeter  may  he  determined  is  car- 
ried out  as  follows: 

After  the  value  of  81  _|-  82  is  obtained,  the  coupling  between  the  wavemeter  coil  and  the 
antenna  system  must  not  be  altered.  A  piece  of  resistance  wire,  R  (Fig.  217),  is  stretched 
tightly  between  two  binding  posts  and  connected  in  series  with  the  circuit  of  the  wavemeter. 
The  amount  of  wire  in  use  is  gauged  by  the  sliding  contact,  T-l.  (A  piece  of  No.  28  German 
silver  wire  or  Tberlo  wire  about  15  inches  in  length  will  be  found  satisfactory.) 

With  the  pointer  of  the  condenser  set  at  Cr,  or  resonance,  the  spark  gap  is  energized  and 
resistance  added  at  R  until  the  reading  of  the  wattmeter  falls  to  exactly  one-half  that 
obtained  in  the  original  resonance  adjustment,  or  0.04  watt.  The  condenser  of  the  wave- 
meter  is  then  shifted  to  either  side  of  resonance  to  such  a  value  of  capacity  that  will  give 


^ggj 


Fig.  222 — Rear  View  of  Kolster  Decremeter. 

one-half  the  wattmeter  reading  obtained  at  Cr,  viz.,  0.02  watt.  Let  the  capa'city  of  the  wave- 
meter  condenser  below  resonance  by  represented  by  C3  and  the  capacity  above  resonance  by 
C«.  Then  the  following  formula  is  applicable : 


_|_  82  _j-  5s  — 


X  1.57 


(6) 


Now,  8S  is  the  decrement  due  to  the  addition  of  the  resistance  R,  and  it  is  evident  that  if 
the  value  of  81  -|-  82  is  subtracted  from  81  _}_  Sa  _|_  Sa,  the  value  of  &  is  at  once  obtained. 

It  has  been  shown  by  Fleming  and  others  that  if  the  value  of  82  is  thus  found  out,  we  may 
evaluate  the  decrement  of  the  wavemeter  82  by  the  following  formula : 

Letting  V  stand  for  the  value  obtained  in  equation  No.  5  and  Vi  for  the  value  in  No.  6, 
then, 

V1  X  5s 

*=-  (?) 

2V  —  Vi 


204 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Hence,  by  subtracting  the  value  of  82  from  5i  -j-  6-2  We  have  5i,  the  decrement  of  the  aerial 
circuit  under  test. 

Assume  for  example, 


and 


52 


-f-  52  =  0.15 
_j_  5a=0.17 

thenSs  =  0.02 
0.17 

and  5s  =  — 


X  0.02 


=  0.026 


0.30  —  0.17 
therefore  Si  =0.15  —  0.026  =  0.124 

If  a  milliammeter  is  connected  directly  in  series  with  the  wavemeter  or  decremeter,  in 

1 
place  of  the  wattmeter,  the  reading  of  the  meter  at  the  point  off  resonance  should  be  - 

1.41 
of  that  obtained  at  resonance  or  approximately  71  per  cent. 

A  resonance  curve  of  the  antenna  oscillations,  having  been  taken  for  any  reason  whatever, 
may  be  used  for  calculation  of  the  decrement,  for  it  is  apparent  that  the  data  required  for 
formula  No.  5  is  included  in  such  curves. 

If  the  variable  condenser  of  the  wavemeter  is  constructed  so  that  the  capacity  varies 
directly  with  the  scale  reading,  the  actual  capacity  of  the  condenser  need  not  be  known. 
The  reading  of  the  condenser  scale  for  the  points  on  and  off  resonance  can  be  substituted  in 
formula  No.  5  in  place  of  the  condenser  capacity  and  the  decrement  measurement  carried  on 
as  usual. 

Formula  No.  2  can  be  written  : 


\2   —    X. 


Si  -f  52  =r 


3.1416 


Xr 


(8) 


where  Xr  =  wave  length  at  resonance ; 

1 1 -  —  current  corresponding  thereto  in  the  circuit  of  the  wavemeter; 

X2  ~  wave  length  3  per  cent  to  5  per  cent,  above  resonance ; 

Xi  —  wave  length  3  per  cent,  to  5  per  cent,  below  resonance ; 

I2  =  current  corresponding  to  wave  lengths  above  and  below  resonance. 

The  decrement  of  the  wavemeter  must,  of  course,  be  subtracted  to  obtain  the  value  of  Si. 

Direct  reading  decremeters,  such  as  the  Kolster*  decremeter  (U.  S.  Bureau  of  Standards) 
are  in  use.  In  this  instrument,  a  dial  geared  to  the  movable  plates  of  the  variable  condenser 

is  fitted  with  a  scale  of 
decrements  and  when  the 
reading  of  the  wattmeter 
at  a  given  point  off 
resonance  is  one-halt 
of  that  obtained  at  reson- 
«ince,  the  combined  decre- 
ment of  the  decremeter 
and  the  circuit  under 
measurement  is  obtained 
by  first  setting  the  decre- 
ment scale  at  zero  (when 
one-half  resonance  cur- 
rent is  obtained)  after 
which  the  decrement  dial 
is  clamped  to  the  movable 
plates  of  the  variable 
condenser  and  the  latter 
turned  to  resonance  and 
to  a  point  beyond  where 
the  reading  of  the  watt- 
meter again  falls  to  one- 


Fig.  223 — Wavemeter  as  a  Source  of  Radio  Frequency  Oscillations. 


'Bureau   of  Standards  publication   No.   235   describes  the  Kolster  Decremeter  in   detail. 


PRACTICAL  RADIO  MEASUREMENTS. 


205 


half  that  at  resonance.    The  decrement  dial  will  then  indicate  the  combined  decrement  of  the 
circuit  under  measurement  and  that  of  the  decremeter. 

A  photograph  of  the  Marconi  decremeter  appears  in  Fig.  220  and  the  Kolster  decremeter 
in  Figs.  221  and  222.  The  latter  is  used  by  all  U.  S.  Government  radio  inspectors. 

171.  Wavemeter  as  a  Source  of  High  Frequency  Oscillations. — A  wave- 
meter  excited  by  an  ordinary  vibrating  buzzer  may  be  used  to  generate  feeble 
oscillations  of  radio-frequency  for  measuring  purposes. 

The  complete  diagram  for  the  apparatus  is  shown  in  Fig.  223.  The  circuit  from  the 
battery  to  the  buzzer  B-l  is  completed  through  the  coil  of  the  wavemeter  L.  To  eliminate 
sparking  at  the  vibrator,  the  windings  (of  the  buzzer)  are  shunted  by  a  non-inductive  re- 
sistance of  100  ohms  or  by  a  condenser  of  1  mfd.  capacity. 

When  the  buzzer  is  put  in  action,  a  change  in  the  lines  of  force  threading  through  L 
takes  place  and  the  condenser  C  receives  a  charge,  which  afterward  discharges  through  L 

1 
at  a  frequency  corresponding  to  —     —      Since  the  capacity  of  C  is  variable,  oscillations  of 

V  L  C  ' 

various  frequencies  may  be  generated  and  caused  to  act  inductively  on  another  circuit  of 
radio-frequency  for  whatever  purpose  required. 


Fig.  224 — Station  Type  Wavemeter  of  the  American   Marconi  Company,   Showing  Exploring  Coil,   Thermo- 
couple  and    Galvanometer. 

172.  Calibration  of  the  Secondary  and  Primary  Circuits  of  a  Receiving 
Tuner. — The  secondary  circuit  of  a  receiving  tuner  can  be  calibrated  in 
terms  of  wave  length  by  the  method  shown  in  Fig.  223,  wherein  the  secondary 
winding  of  a  tuner  is  indicated  at  L-l,  the  shunt  variable  condenser  at  C-l,  the 
detector  at  D,  and  the  telephone  at  P. 

A  certain  value  of  inductance  and  capacity  are  selected  at  L-l  and  C-l  respectively, 
followed  by  setting  the  buzzer  into  vibration.  With  the  wavemeter  in  inductive  relation,  the 
capacity  of  the  wavemeter  condenser  is  varied  until  a  maximum  of  sound  is  heard  at  tele- 
phone P.  The  wave  length  of  the  secondary  circuit  is  then  identical  with  the  setting  of  the 
wavemeter. 


206 


PRACTICAL  WIRELESS  TELEGRAPHY. 


Other  values  of  inductance  may  be  selected  at  L-l,  the  capacity  of  C-l  varied  progressively 
from  zero  to  maximum,  and  the  corresponding  wave  length  measured  by  the  wavemeter. 
These  calibrations  may  be  plotted  in  curve  form  on  cross-section  paper  or  in  the  form  of  a 
table  for  ready  reference. 

The  open  circuit  of  a  receiving  station  can  be  calibrated  by  the  method  shown  in  Fig. 
213.  With  the  buzzer  in  operation,  inductance  is  progressively  added  at  the  aerial  tuning 
inductance  L-2  or  the  primary  winding  L-l  of  the  receiving  transformer,  and  the  wave 
length  reading  observed  at  the  wavemeter.  The  reduction  of  wave  length  occasioned  by  the 
short  wave  condenser  may  be  measured  on  the  wavemeter  in  the  same  manner. 

If,  during  the  preliminary  adjustment,  resonant  response  cannot  be  secured  by  this  method, 
it  indicates  that  the  wavemeter  has  not  the  proper  range  for  the  tuner.  Hence,  some  knowl- 
edge of  the  probable  wave  length  of  the  circuit  should  be  obtained  before  undertaking  the 
test  and  a  fitting  wavemeter  supplied.  The  wave  length  can  be  roughly  estimated  from  the 
approximate  values  of  inductance  and  capacity  in  the  circuit. 


Buzzer-  Tester  ~== 


Fig.    225 — Showing    How    Open    and    Closed    Oscillation    Circuits    of    a    Receiving    Tuner    May    Be 

Calibrated. 

173.  Calibration  of  the  Open  and  Closed  Circuits  Simultaneously. — The 
most  satisfactory  method  of  calibrating  a  receiving  tuner  or  adjusting  the  re- 
ceiving system  to  a  standard  wave  length  with  a  given  aerial,  is  to  place  a  wave- 
meter  in  inductive  relation  to  the  antenna  at  all  times  as  in  Fig.  225. 

Since  a  change  of  coupling  changes  the  tuning  of  the  open  and  closed  circuits,  the  exact 
adjustment  of  the  circuits  for  the  loudest  response  with  any  degree  of  coupling  can  be  de- 
termined by  setting  the  wavemeter  into  excitation  and  causing  it  to  act  inductively  upon  the 
open  circuit.  This  should  be  followed  by  variation  of  the  inductance  and  capacity  in  both 
the  open  and  closed  circuits  until  a  sharp  maximum  is  obtained  in  the  head  telephones  of  the 
receiver.  The  loudest  response,  of  course,  is  secured  when  the  wavemeter,  the  closed,  and 
the  open  circuit  are  all  in  electrical  resonance.  At  the  same  time,  the  crystal  rectifier  can 
be  adjusted  to  maximum  sensitiveness. 

Care  should  be  taken,  during  this  calibration,  to  vary  the  values  of  inductance  and  capacity 


PRACTICAL  RADIO  MEASUREMENTS. 


207 


of  the  closed  and  open 
circuits  in  a  progressive 
manner,  e.  g.,  an  increase 
of  inductance  in  the  an- 
tenna circuit  calls  for  a 
corresponding  increase 
in  the  closed  or  detector 
circuit,  or  vice  versa. 

Using  solid  rectify- 
ing detectors,  the  in- 
ductance of  the  wave- 
meter  can  only  be  placed 
an  inch  or  so  from  the 
earth  lead,  but  with  a 
sensitive  vacuum  valve 
amplifier,  the  wave- 
meter  may  be  placed 
several  feet  from  that 
lead. 


L      WAVEMETER 


Fig.    22£ — The    Measurement    of    the    Natural    Wave    Length    of   a    Simple 
Tuning    Coil. 


174.  Measurement    of   the    Natural    Oscillating    Period   of    a   Coil. — The 

natural  wave  length  of  a  tuning  coil,  as  a  simple  open  circuit  oscillator,  may  be 
measured  by  the  circuit  and  apparatus  shown  in  Fig.  226. 

The  radio-frequency  coil  A,  B,  is  set  into  excitation  by  an  aperiodic  buzzer  circuit  which 
includes  the  1  mfd.  condenser  C-l  and  the  resistance  R.  of  2,000  ohms.  The  wavemeter,  fitted 
with  a  sensitive  detector 
D,  is  placed  in  inductive 
relation  to  the  coil  in  the 
usual  manner.  If  the 
coil  possesses  a  fair 
value  of  distributed  ca- 
pacity, the  resonant  point 
on  the  wavemeter  will 
be  sharply  defined,  but, 
lacking  this,  the  point  of 
resonance  may  be  some- 
what difficult  to  locate. 

175.  Measurement 
of  Electrostatic  Ca- 
pacity.—  (a)  Capac- 
ity at  low  voltage  and 
audio-frequencies.  We 
may  measure  the  sim- 
ple electrostatic  ca- 
pacity of  a  condenser 
or  an  aerial  by  the 
bridge  method  of  Fig. 
227.  The  condenser 
of  unknown  capacity 
is  shown  at  C-x,  and 
a  standard  variable 
condenser  calibrated 
in  microfarads  at  C-n. 
R-l  and  R-2  are  non- 
inductive  resistances 
of  low  value  such  as 
found  in  the  usual 
sliding  .wire  bridge.  P  rig.  227— Diagram 


Electrostatic    Capacity. 


208 


PRACTICAL   WIRELESS   TELEGRAPHY. 


and  S  are  the  primary  and  secondary  windings  respectively  of  an  ordinary  tele- 
phone induction  coil.  A  vibrating  buzzer  is  indicated  at  B-l,  and  the  battery  at 
B-2.  This  apparatus  affords  a  source  of  electromotive  force  for  obtaining  balance 
of  the  bridge. 

To  carry  out  the  measurement,  the  buzzer  is  set  into  vibration  and  the  values  of  R-l  and 
R-2.  varied  until  complete  silence  or  a  minimum  of  sound  is  heard  in  the  telephone  P-l.  The 
various  quantities  are  then  related  as  folows : 

C-x        R-2  R-2 

= or  C-x  = X  C-n 

C-n        R-l  R-l 

If  condensers  C-x  and  C-n  have  different  dielectrics,  complete  silence  cannot  be  obtained, 
hence  a  minimum  of  sound  is  an  indication  of  balance.  To  carry  out  the  measurement  for 
low  values. of  capacity,  the  total  resistance  of  the  sliding  wire  bridge,  need  not  exceed  110  ohms. 

If  the  aerial  and  earth 
wire  of  an  antenna  sys- 
tem be  substituted  for  the 
condenser  C-x,  its  elec- 
trostatic capacity  may 
thus  be  determined,  but 
it  should  be  borne  in 
mind  that  the  simple 
electrostatic  capacity  of 
an  aerial  and  its  "ef- 
fective" capacity  when 
traversed  by  alternating 
current  of  radio-fre- 
quency are  of  different 
value.  This  is  due  to  the 
non-uniform  distribution 

Fig.    228— Measurement    of    Electrostatic    Capacity    at    Radio    Frequencies.        JJ*     cu™"ent     and     voltage 

throughout  the  open  cir- 
cuit which  causes  the  capacity  to  vary  with  the  applied  frequency.  Generally  the  effective 
capacity  is  less  than  the  simple  electrostatic  capacity.  (The  latter  value  is  the  capacity 
obtained  by  considering  the  aerial  merely  as  one  plate  of  a  condenser,  the  earth  the  opposite 
plate  and  with  uniform  current  and  voltage  distribution.) 

A  standard  of  capacity  for  the  bridge  may  be  constructed  from  flat  brass 
plates,  of  rectangular  form  and  its  capacity  may  be  calculated  by  the  following 
formula : 

A 

C  =  -  mfds. 

47r  T  900,000 
where  A  =  area  in  sq.  cms.  of  the  dielectric  covered  by  the  plates, 

T  =  separation  of  the  plates  in  cms. 

(b)  Measurement  of  Capacity  at  Radio-Frequencies.  The  capacity  of  the 
condensers  used  in  receiving  circuits  may  be  determined  by  the  method  shown  in 
diagram  of  Fig.  228.  A  wavemeter  L,  C  (or  any  variable  condenser  and  induct- 
ance within  the  required  range)  is  set  into  excitation  by  the  buzzer  B-2.  The 

resulting  oscillations  are  caused  to 
act  upon  the  second  circuit  of  vari- 
able frequency,  L-l,  C-x  or  L-l, 
C-n.  C-n  is  a  standard  variable  con- 
denser calibrated  in  microfarads, 
and  C-x  the  condenser,  the  capacity 
of  which  is  to  be  determined.  A 
crystal  rectifier  connected  uni-later- 
ally  at  D  is  shunted  by  the  telephone 
P.  A  double  pole  double  throw 
switch  permits  either  condenser  to 

rig.    229 — Measurement    of    Capacity    by    Means    of    a          i  j  •    j.      ,1 

Wavemeter.  be  connected  into  cri 


PRACTICAL  RADIO  MEASUREMENTS.  209 

With  the  huzzer  in  vibration,  inductance  L-l  is  varied  until  resonant  response  is  obtained 
in  the  telephone  or  when  L,  C  :=  L-l,  C-x.  The  D.  P.  D.  T.  switch  is  then  thrown  to  C-n, 
and  its  capacity  altered  until  circuit  L-l,  C-n,  is  again  in  resonance  with  L,  C.  Obviously, 
the  capacity  of  C-x  is  that  of  C-n,  which  is  already  known. 

For  accuracy,  the  coupling  between  L  and  L-l  should  be  reduced  to  a  degree  consistent 
with  the  strength  of  signals.  To  bring  the  two  circuits  in  resonance,  it  may  be  necessary 
with  certain  condensers  to  vary  the  capacity  of  C. 

Another  method  for  quickly  determining  the  capacity  of  a  condenser  is  shown  in  Fig.  229. 
A  standard  wavemeter,  L,  C-n,  has  the  condenser  of  known  capacity  C-n  and  the  crystal 
rectifier  D  connected  unilaterally.  The  wavemeter  is  tuned  to  resonance  with  any  spark 
transmitter  such  as  L-l,  C-l,  S-l,  and  the  capacity  of  C-n  noted.  With  the  spark  still  dis- 
charging at  S-l,  C-x  is  connected  in  shunt  to  C-n  and  the  capacity  of  C-n  reduced  until 
resonance  is  obtained  again.  For  example,  if  the  capacity  of  C-n  in  the  first  measurement  is 
.007  mfds.,  and  in  the  second  test,  .002  mfds.,  the  capacity  of  C-x  must  be  the  difference  of  the 
two  readings,  or  .005  mfds.  Circuit  L-l,  C-l,  S-l  may  be  replaced  by  a  wavemeter  and  buzzer. 

176.  Measurement  of  the  Effective  Inductance  of  a  Coil  at  Radio 
Frequencies.  —  The  effective  inductance  of  a  given  coil  can  be  determined  by 
the  connections  and  arrangement  of  apparatus  in  Fig.  228.  Assume  that  the  in- 
ductance of  L-l  is  required  and  condenser  C-n  is  connected  in  shunt;  then  L,  C, 
and  L-l,  C-n,  are  tuned  to  resonance  with  very  loose  coupling.  Then  if  L.C,  is  a 
standard  wavemeter,  the  wave  length  of  L-l,  C-n  is  at  once  obtained.  Then, 

A2 


3,552  X  C-n 

where  L  =  the  inductance  of  the  coil  in  centimeters. 

177.  Calculation  of  Inductance  from  the  Constants  of  the  Coil.*—  The 
Nagaoka  formula  for  calculating  the  inductance  of  a  single  layered  coil  is 
expressed  as  follows  :  ^  ns 

L  =  4  »*  -  K, 

b 
where  L  =  inductance  in  centimeters, 

a  =  mean  radius  of  coil  in  centimeters, 

n  =  total  number  of  turns, 

b  =  length  of  coil  in  centimeters, 

2  a 
K  =  a  constant  varying  as  the  ratio  -  . 

2a  b 

Values  of  K  for  —  appears  in  the  curves  of  Fig.  230. 

b 

If  the  dimensions  of  the  coil  are  given  in  inches,  the  student  can  convert  them  to  centi- 
meters by  multiplying  by  2.54  (1  inch  =  2.54  centimeters).     Also  the  radius  is  one-half  the 

2a 
diameter,  hence  the  ratio  of  —  is  readily  obtained,  and  the  corresponding  value  of  K  from  the 

b 
2a 
curve.     Thus  if  —  =  1.35,  then  K  =  0.84. 

b 

(a)  Example  1.  —  Assume  a  coil  of  10  turns  with  mean  diameter  of  18  inches,  and  overall 
length  including  the  insulation  (if  any)  of  12.5  inches.  The  inductance  is  calculated  as  follows  : 
n  =  10  turns  ; 
b  =  31.75  centimeters; 
a  =  22.9  centimeters  ; 
2a      18 

=  1.44  and  from  Fig.  230  K  =  0.826. 
b       12.5 

(22.9)  3  X   (10)* 

Therefore  L  =  4**  ---     -  X  0.826  or  53,825  centimeters. 
31.75 


'Presented   by   Wm.   II.   Priess  in  the  August,   1915,  issue  of  the  Wireless  Age. 


210 


PRACTICAL   WIRELESS   TELEGRAPHY. 


PRACTICAL  RADIO  MEASUREMENTS. 


211 


(&)  Example  2.— Calculate  the  inductance  of  a  coil  three  inches  in  diameter,  8  inches  in 
length,  wound  closely  with  No.  26  double  silk-covered  wire.  From  the  table  on  page  213  the. 
diameter  of  No.  26  D.  S.  C.  wire  is  .02014  inches,  hence, 


8 


=  397  turns ; 


0.02014 
b  —  20.3  centimeters ; 
a  =  3.81  centimeters. 
2a 

_  —  3/8  —  0.375,  and  from  Fig.  196 
b 
K  =  0.859 

(3.81  r  X  (397) 2 

Therefore  L  =  4ir  -  -  x  0.859,  or  3,820,000  centimeters. 

20.3 


i.oo 


09O 


O  IQ 


0  00 

O6       04      Oi       oo^Oi       04-      06      03        10       li         14        16        18       20 

Fig.  231 — Curves  Showing  the  Value  of  a  Constant  for  the   Ratio  of  the  Diat 


lHf«1W*"ff»~'yir"y.i  ^  3.4 

eter  of  a  Wire  to  Its  Pitch. 


In  the  curves  of  Fig.  230,  values  of  K  are  given  for  coils  lying  between  those  of  infinite 
length  and  those  where  the  diameter  is  9l/2  times  the  length.  This  covers  all  conditions 
encountered  in  practice. 

(c~)  Correction  factor. — For  practical  calculation  the  foregoing  formula  is  sufficiently 
accurate,  but  for  greater  precision  a  correction  factor  must  be  applied.  'In  fact,  the  inductance 
values  obtained  in  examples  1  and  2  are  termed  the  ''current  sheet''  inductance,  based  upon 
the  assumption  of  a  coil  wound  with  infinitely  thin  tape,  the  turns  of  which  are  assumed  to 
touch  but  not  to  be  in  electrical  contact. 

Dr.  Rosa  of  the  Bureau  of  Standards  has  given  an  expression  for  circular  sectioned  non- 
magnetic wire  as  follows :  If  the  true  inductance  is  designated  as  L,  and  the  current  sheet 
inductance  as  L-s,  then, 

L  =  L-s  —  L-c 
where  L-c  is  the  correction  value,  and, 

L-c  —  4^r  a  n   (K->  -f  K») 

where  Ka  and  K?  are  constants  of  the  coil.     (See  B    of  S     vol   8,  pp   197  and  199) 

d 

Now  K*  is  a  constant  plotted  to  the  parameter  of  —  which  is  the  ratio  of  the  diameter 


PRACTICAL  WIRELESS  TELEGRAPHY. 


COO  0- 

Fig.   232— Showing   the   Values    of 


1.0  0.15  O2.0  0-25  0.30 

a   Correction   Factor   for   Determining  the   True   Inductance    of  a  Coil 


PRACTICAL  RADIO  MEASUREMENTS. 


213 


of  bare  wire  to  its  pitch  in  a  given  winding  and  for  any  given  value  of  —  a  value  of  Ka 

D 

appears  in  Ing.  231,     It  should  be  noted  that  Kz  may  be  positive,  negative  or  zero;  that  is, 
on  the  left  of  the  zero  line  Ka  is  positive  and  on  the  right  hand  side  negative. 

Ka  is  another  factor  plotted  against  the  number  of  turns  in  the  coil  as  in  Fig.  232  where 
it  will  be  seen  that  Ks  is  zeio  for  one  turn  and  0.3365  for  1,000  turns. 

(d)  Corrections  for  Example  1. — We  shall  now  calculate  the  value  of  L-c  for  example  1. 

d 
Assuming  the  wire  of  the  helix  to  have  a  diameter  of  *4  inches  with  spacing  of  one  inch,  — 


.250 


1) 


—  .025  and  from  the  curve,  Fig.  231,  Ka  —  — 0.8  (of  negative  value). 


1.000 

Since  N  —  10,  then  from  the  curve,  Fig.  232,  Ka  =  0.266,  hence, 
L-c  =  4*-  x  22.9  X  10  (—0.8  +  0.266), 
L-c  =  12.566  X  22.9  X  10  (—0.534)  cms. 
hence  L-c  =  — 1,540  cms. 

and  L  =  53,825  —    (—1,540)    =   55,365  cms. 

(e)  Correction  for  Example  2. — The  diameter  of  bare  No.  26  B.  and  S.  wire  is  .01790 
inches  and  of  D.  S.  C.  No.  26,  .02014  inches,  hence, 

d        .01790 

—  = =  0.889 

D       .02014 

From  Ka  curve,  Ka  =  -fO.441  and  since  n  =  397,  therefore  Ka  =  0.335  approx. 
Therefore  L-c  =  4*  X  3.81  X  397  (0.441  +  0.335)   cms. 
L-c  — 14,750  cms. 

L  =  3,820,000  —  14,750  =  3,805,250  cms. 
L-c 
and  —  =  0.38  per  cent,  the  correction  factor. 

L  • 

The  correction  factor  is  thus  quite  appreciable  for  some  coils  and  rather  negligible  for 
others.  With  currents  of  radio-frequency  there  is  an  altered  current  distribution  in  the  con- 
ductor which  affects  the  inductance,  giving  a  lesser  value  than  in  circuits  where  the  current 
is  uniformly  distributed  throughout,  but  the  finer  the  wire,  the  smaller  will  be  the  change  of 
inductance  due  to  increase  of  frequency. 

A  table  of  the  diameter  of  various  sized  wires  in  the  B.  and  S.  gauge  follows : 

TABLE  NO.  1. 

Silk  and  Cotton-Covered  Annealed  Copper  Wire. 
Diameter  in  Mils. 


B.&S. 

Single 

Double 

Single 

Double 

Gauge 

Bare 

Cotton 

Cotton 

Silk 

Silk 

20 

31.961 

37.861 

42.161 

34.261 

36.161 

21 

28.462 

34.362 

38.662 

30.762 

32.662 

22 

25.347 

31.247 

35.547 

27.647 

29.547 

23 

22.571 

28.471 

32.771 

24.871 

26.771 

24 

20.100 

26.000 

30.300 

22.401 

24.300 

25 

17.900 

23.800 

28.100 

20.200 

22.100 

26 

15.940 

21.840 

26.140 

18.240 

20.140 

27 

14.195 

20.095 

24.395 

16.495 

18.395 

28 

12.641 

18.541 

22.841 

14.941 

16.841 

29 

11.257 

17.157 

21.457 

13.557 

15.457 

30 

10.025 

15.925 

20.225 

12.325 

14.225 

31 

8.928 

14.828 

19.128 

11.228 

13.128 

32 

7.950 

13.850 

18.150 

10.250 

12.150 

33 

7.080 

12980 

.17.280 

9.380 

11.280 

34 

6.304 

12.204 

16.504 

8.504 

10.504 

35 

5.614 

11.514 

15.841 

7.914 

9.814 

36 

5.000 

10.900 

15.200 

7.300 

9.200 

37 

4.453 

10.353 

14.653 

6.753 

8.653 

38 

3.965 

9.865 

14.165 

6.265 

8.165 

39 

3.531 

9.431 

13.731 

5.831 

7.731 

40 

3.144 

9.044 

13.344 

5.344 

7.344 

214 


PRACTICAL  WIRELESS   TELEGRAPHY. 


This  data  is  essential  in  the  calculation  of  the  inductance  of  the  coils  in  radio-frequency 
circuits,  particularly  the   tuning  coils   of  the   receiving  apparatus.     Knowing  the  length   of 

the  winding  and  the  diameter  of  the  wire, 
the  total  number  of  turns  is  obtained  in 
dividing  the  former  by  the  latter.  Multi- 
plying this  by  the  circumference  of  the 
coil,  the  number  of  feet  of  wire  required 
for  a  given  winding  is  at  once  obtained. 

178.  Measurement  of  the  Ef- 
fective Inductance  and  Capacity 
of  an  Aerial.  —  To  measure  the  ef- 
fective inductance  of  an  aerial,  a 
standard  wavemeter  and  a  standard 
of  inductance  is  required.  The  latter 
can  be  made  up  of  Litzendraht  wire 
and  calculated  from  the  inductance 
formula  in  the  foregoing  paragraph. 
The  aerial  is  set  into  excitation  by  an 
induction  coil  or  transformer  S,  Fig.  233, 
and  the  wavemeter  placed  in  inductive 
relation.  The  reading  of  the  natural  wave 
length  thus  obtained  may  be  designated  as 
Xi.  We  next  insert  the  standard  induct- 
ance L1  and  take  a  second  reading  of  the 
wave  length  which  may  be  designated 
by  Xs. 


PHONES 

Fig.   233 — Diagram   of  Connections  for  Measuring   the 
Inductance    and    Capacity    of   an    Aerial. 


then  L  =  -  $  — 

(\22  —  X!2) 

where  L  =  inductance  of  the  aerial  in  microhenries  ; 

L1  —  inductance  of  the  standard  in  microhenries. 

To  determine  the  effective  capacity  of  an  aerial,  first  measure  the  natural  wave  length  Xi 
then  insert  a  condenser  C1  of  .001  microfarads  capacity  and  take  a  second  measurement  of 
wave  length  X2.  Obviously  X2  is  less  than  Xi.  Then  the  capacity 


— 

— 


Xi2 


where  C  =  capacity  of  the  aerial  in  microfarads. 

179.  Calibration  of  a  Wavemeter  from  a  Standard. — If  a  calibrated  wave- 
meter  can  be  procured,  the  wave- 
meter  described  at  the  beginning  of 
this  chapter  can  be  calibrated  from 
it  in  a  simple  manner,  as  shown  in 
Fig.  234. 
tively,  are 


Here,  L  and  C  respec- 
the  inductance  coil  and 
condenser  of  a  standard  wavemeter 
which  is  set  into  excitation  by  the 
buzzer,  H,  and  the  batteries  B.  The 
winding  of  the  magnets  of  the 
buzzer  are  shunted  by  a  condenser 
K,  of  about  1  microfarad  capacity, 
for  which  may  be  substituted,  if 
desired,  a  non-inductive  resistance 
of  about  100  ohms.  These  are  in- 
tended to  absorb  the  counter  electromotive  force  of  the  buzzer  winding. 

When  the  buzzer  is  set  into  operation,  the  wavemeter  L  C  becomes  a  miniature  trans- 
mitting set,  radiating  waves  corresponding  to  a  definite  frequency  of  oscillation  which  will 
be  recorded  on  the  wavemeter  L-l,  C-l,  when  it  is  in  resonance  with  L,  C.  The  standard 


Fig.    234— Simple 


Method    of    Calibrating 
from    a    Standard. 


Wavemeter 


PRACTICAL  RADIO  MEASUREMENTS. 


215 


wavemeter  L,  C  is  then  set  at  various  wave  lengths  and  the  buzzer  put  into  operation,  being 
carefully  adjusted  for  clear  tones.  The  capacity  of  the  condenser  C-l,  is  then  altered  until 
a  maximum  of  sound  is  heard  in  the  head  telephones.  Obviously,  L-l,  C-l  has  the  same 
wave  length  as  L,  C,  and  a  record  of  the  setting  is  made  accordingly.  Thus  if  six  or  eight 
readings  are  taken,  covering  the  entire  scale  of  C-l,  the  data  can  be  plotted  on  cross-section 
paper  in  the  form  of  a  curve.  Intermediate  values  of  wave  lengths  are  readily  determined 
from  the  curve. 

For  accuracy  during  calibration,  the  degree  of  coupling  between  L  and  L-l  must  be  kept 
as  low  as  is  consistent  with  the  strength  of  signals.  If  response  is  not  secured  readily  at 
the  wavemeter,  L-l,  C-l,  it  may  be  that  the  values  of  inductance  and  capacity  are  such  that 
the  circuit  is  out  of  resonance  with  the  standard  wavemeter  L,  C.  If  so,  different  values 
of  inductance  or  capacity  must  be  selected  until  a  resonant  response  is  secured.  It  is  not 
necessary  for  the  crystal  rectifier,  shown  in  Fig.  234,  to  be  connected  unilaterally  to  the 
wavemeter.  It  may  be  shunted  around  the  circuit  as  in  the  usual  receiving  set. 


Fig.    235 — Three    Way    Calibration    of    Wavemeter    for    Greater    Accuracy. 

To  eliminate  all  possibility  of  error  in  calibration  due  to  the  added  capacity  effect  of  the 
shunt  excitation  circuit  of  the  foregoing  connection,  a  three-way  method  may  be  employed 
as  shown  in  Fig.  235.  Herein,  B  is  an  accurately  calibrated  standard  wavemeter  with  a 
crystal  detector  and  head  telephones  connected  unilaterally.  The  wavemeter  under  calibra- 
tion is  represented  at  C  and  also  has  a  detector  connected  unilaterally.  At  A  are  the  circuits 
comprising  the  fixed  inductance  L,  the  condenser  C,  the  buzzer  B,  the  battery  Bat,  and  the 
shunt  condenser.  By  means  of  the  calibration  chart  furnished  with  the  wavemeter  B,  the 
condenser  C-l  can  be  set  at  any  wave  length  within  the  range  of  the  meter. 

The  buzzer  having  been  put  into  operation,  the  capacity  of  the  variable  condenser  of  tb' 
oscillation  circuit,  A,  is  changed  until  maximum  response  is  secured  at  the  head  telephones 
of  the  wavemeter  B.  During  this  operation  the  coupling  between  the  coils,  L  and  L-l,  should 
be  as  loose  as  is  consistent  with  the  strength  of  signals  in  the  head  telephones.  The  point  of 
resonance  having  been  determined,  the  coil  L-2,  of  the  wavemeter,  C,  is  placed  in  inductive 
relation  to  L.  The  capacity  of  the  condenser,  C-2,  is  then  altered  until  a  resonant  response 
is  secured  in  the  head  telephones.  Obviously,  the  wave  length  of  the  wavemeter  C,  is  now 
identical  with  that  of  the  wavemeter  B. 


216 


PRACTICAL  WIRELESS   TELEGRAPHY. 


This  process  is  repeated  until  a  complete  set  of  calibrations  are  obtained.  If  desjred, 
the  crystalline  detectors  connected  to  the  wavemeters,  B  and  C,  may  be  connected  in  series 
with  the  head  telephone  and  then  in  shunt  to  the  condenser.  The  connection  shown,  how- 
ever, affords  greater  accuracy  because  the  calibration  of  the  wavemeter  is  not  so  seriously 
affected. 

180.  Measurement   of    Mutual    Inductance   at    Radio  /Frequencies. — As- 
sume that  two  coils  such  as  the  primary  and  secondary  windings  of  a  receiving 

tuner,  L-2  and  L-3,  Fig.  236,  are  in 
inductive  relation  and  the  mutual  in- 
ductance is  required. 

With  the  binding  posts  A,  B  and  C,  D 
connected  together  by  a  jumper,  the 
C-i  wavemeter  is  tuned  to  resonance  with  the 
spark  gap  circuit,  L,  C,  S.  The  capacity 
of  the  wavemeter  condenser  C-l  is  ob- 
served and  may  be  designated  as  C-2. 
Inductances  L-2  and  L-3  are  now  con- 
nected in  series  and  a  new  value  of  C-l 
obtained  for  resonance,  designated  as 

Fig.    236— Showing    the    Connections    for    Measuring  C~3'      Then.  when    the    magnetic    fields    of 

the   Mutual    Inductance   of   Two   Coils   at  the  two  coils  are   in  the   same  direction, 

Radio    Frequencies.  a    jarge    yalue    Qf    inductance    is    obtained 

which  is  equal  to  L-2  -f-  L-3  -f-  2M,  and  in  terms  of  the  two  condenser  capacities  observed 
on  the  wavemeter, 


L-2  -f  L-3  -f  2M  - 


L-l 


where  L-l  =  inductance  of  the  wavemeter  in  microhenries. 

After  this  value  is  obtained,  the  connections  from  L-3  to  C,  D  are  reversed  (as  shown 
by  the  dotted  lines)  and  the  measurement  gone  through  again.  Then  if  this  is  the  smaller 
value  of  inductance, 

C-2 
L-2  +  L-3  —  2M  =    I 1   \  L-l 


-  2M  =    ( 

V    C-4 


Where  C-4  =  capacity  of  the  condenser  at  resonance  with  the  connections  to 
C,  D  reversed. 

The  two  values  of  inductance  having  thus  been  obtained,  the  mutual  inductance 

L-5  — L-6 

M  = 

4 

Where  M  =  the  mutual  inductance  in  microhenries ; 
L-5  =  larger  value  of  total  inductance ; 
L-6  =  smaller  value  of  total  inductance. 

181.  Comparative  Measurement  of  the  Strength  of  Incoming  Signals.— 
The  human  ear  is  not  to  be  relied  upon  to  judge  the  strength  of  incoming  signals 
at  a  given  receiving  station  when  they  vary  in  intensity  by  values  less  than  25 
per  cent.  an$  since  the  comparison  of  the  signals  from  one  or  more  transmitting 
stations  is  often  desirable  for  purposes  of  record  or  adjustment,  more  accurate 
means  must  be  provided.  - 

The  ideal  way  to  measure  the  strength  of  the  incoming  signals  would  be  to  place  a 
galvanometer  in  the  receiver  circuits,  but  this  is  not  feasible,  particularly  when 
the  receiving  station  is  far  distant  from  the  transmitter  or  in  case  discharges  of 
atmospheric  electricity  are  rather  severe.  Hence  the  measurement  is  carried  out  by  means 
of  a  calibrated  shunt  resistance  connected  across  the  receiving  telephone. 

The  measurement  of  the  signal  intensity  is  carried  out  as  follows :  A  given  transmitter 
is  tuned  to  maximum  strength  of  signals  at  the  receiver,  followed  by  connecting  a  variable 
resistance  across  the  telephones,  the  resistance  being  reduced  until  the  signals  just  disappear 
or  are  barely  audible. 

If  we  designate  the  value  of  the  telephone  current  in  microamperes  required  to  make 
the  least  audible  signal  as  C-a,  it  has  been  shown  that 


PRACTICAL  RADIO  MEASUREMENTS. 


217 


R  +  T 

C=-      -XC-a 
R 

where  C  =  current  in  the  telephone  without  the  shunt ; 

R  =  resistance  of  the  shunt  for  the  least  audible  signal ; 
T  =  resistance  of  the  telephone. 

Now  the  value  of  C-a  for  the  least  audible 
signal  varies  with  the  frequency  of  the 
current  flowing  through  the  telephone,  the 
impedance  of  its  windings,  and  the  sensi- 
tiveness of  the  human  ear  to  weak  sounds. 
Hence  at  a  given  station  the  value  of  C-a 
is  ignored  and  the  strength  of  the  signal 
spoken  of  as  being  so  many  times  audi- 
bility. 

Thus  as  in  the  diagram,  Fig.  237,  if 
the  value  of  R  for  the  least  audible  signal 
is  50  ohms,  and  the  resistance  of  the  tele- 
phone 2,000  ohms,  then 

2,000  +  50 
C  =  -  -  C-a  =  41,  C-a,  e.  g., 


tt 


/wvwwvv — 


50 


Fig.     237 — Fundamental      Circuit     of     the     Audibility 
Meter. 


the  signal  is  41  times  that  required  to  make 
an  audible  sound  in  the  telephone. 
It  is  clear  that  the  results  obtained  by  this  method  vary  at  each  station  according  to  the 
type  of  apparatus  and  the  keenness  of  the  observer,  hence  the  audibility  factor  cannot  be 
used  at  two  different  receiving  stations  to  compare  the  effectiveness  of  a  given  transmitter. 
It  does,  however,  permit  an  approximation  which  is  superior  to  mere  guesswork. 


Fig.   238 — Audibility    Meter    (American    Marconi    Company). 

Shunt  resistance  boxes  are  manufactured  for  this  purpose  and  are  termed  audibility^  meters. 
The  resistance  coil  is  fitted  with  a  multipoint  switch  and  the  contact  studs  calibrated  in  terms 
of  the  audibility  of  the  received  signals  as  compared  to  the  current  corresponding  to  the 
least  audible  signal  with  the  particular  telephone  supplied  with  the  meter.  The  type  manu- 
factured by  the  Marconi  Company  of  America  is  shown  in  Fig.  238. 


218 


PRACTICAL  WIRELESS   TELEGRAPHY. 


A  typical  chart*  of  the  relative  strength  of  signals  received  at  different  hours 
of  the  day  from  the  Marconi  station  at  Ketchikan,  Alaska,  at  the  receiving 
station  at  Astoria,  Ore.,  is  shown  in  Fig.  239.  These  readings  were  taken  on  a 
Marconi  audibility  meter. 

*Curve  A,  Fig.  239,  shows  the  signal  audibility  throughout  the  24  hours  of  the  day  from  Ketchikan 
to  Astoria.  Curve  S  shows  the  intensity  of  atmospheric  electricity.  It  is  interesting  to  note  the  distinct 
rise  in  both  the  signal  and  "static"  intensity  near  to  midnight  and  the  distinct  drop  during  the  early 
npurs  ot  the  morning.  The  rise  of  the  peak  is  found  to  occur  at  approximately  the  same  hour  in 
observations  taken  over  several  months  duration. 


PRACTICAL  RADIO  MEASUREMENTS, 


219 


o.i 


300 


400 


&OQ 


900 


500  600  70Q 

WAVE  LENGTH    IN    METERS 

Fig.    240 — Curves    Showing    the   Effect   on   the   Radiated  Wave    By   Progressive    Reduction    of   the   Coupling 

at    the   Oscillation  Transformer. 

182.  "Tight"  and  "Loose"  Coupling. — The  adjustment  of  the  coupling  of 
a  transformer  is  important  because  if  the  primary  and  secondary  of  the  oscillation  trans- 
former are  closely  coupled,  oscillations  of  two  frequencies  occur  in  the  antenna  circuit 
as  shown  by  the  curve  A,  Fig-.  240.  If  the  coupling  be  reduced  (by  drawing  the  primary 
and  secondary  windings  apart)  and  a  second  resonance  curve  obtained,  the  two  frequencies 
tend  to  merge  into  one  as  shown  by  curve  B,  Fig.  240.  Further  reduction  of  coupling 
brings  about  the  condition  shown  by  curve  C  in  which  the  energy  radiated  is  confined  to 
practically  a  single  frequency  of  oscillation.  The  wave  C  may  be  of  greater  or  lesser 
amplitude  than  waves  A  and  B,  according  to  conditions. 


Fig.   241 — Showing  How   Loose   Coupling  Can   Be 
Obtained   With    an    Auto    Transformer. 


w 


Fig.     242 — Another     Method     for  "Obtaining     Loose 
Coupling   With    an    Auto    Transformer. 


A  popular  opinion  seems  to  exist  that  the  required  loose  coupling  for  radiating  a  pure  and 
sharp  wave  can  only  be  obtained  by  use  of  the  inductively  coupled  oscillation  transformer. 
Contrary  to  this  belief,  however,  a  pure  wave  can  be  obtained  by  other  methods  of  coupling 
as  well.  Take,  for  example,  the  auto-transformer  shown  in  Fig.  241.  By  the  use  of  three 


220 


PRACTICAL  WIRELESS  TELEGRAPHY. 


contact  clips,  the  turns  for  the  aerial  circuit  can  be  included  between  C  and  D  and  for  the 
primary  circuit  between  A  and  B.    With  the  taps  in  this  position,  the  mutual  inductance  is 

reduced  and  the  coup- 
ling 
The 


s    s     CAROUNA 

Date  3:!P:M6  Tuned  by'  *  B.COLLE50N  "" 

TUNING    RECORD 

Call  Letters  .....KGB.  

•  Type  of  Equipment  ..:?..  ^  W.    500  ~  /R  



Aerial  Type  T.  No.  of  Wires— 

_JT  —  Aerial  Natural  vA'  

WAVE-LENGTH    —    3OO    — 

450    —    600 

Primary                                    '  /A 

L-A—    ~-A  Turns 

Secondary 

...4-^.  .4.  V|fe      "                   Aerial  Capacity 

Loading  Coil  (Fixed)       8A  

*[A.^  I?  ^4       "              M'f'ds 

"     (Var.^         3_±     _ 

...3±  _3  ±_     '- 

'Short    Wave   Condenser  J-N.  

PUT  .PUT    In  or  Qut            Condenser 

.        '                                    jars 

Radiation  Quenched            5«Q  

Decrement      ...  r.PrI??  

p,)Q  ,055  300  .._!  _ 

"3 

Radiation  Rotary              _-  ?-P  - 

7.0  IO,Q  Amperes           45°  ~<  

% 
| 

Decrement       ..  ..Or.'.?  

0.10  ,055  _                    too  —  fe  — 

~ 

Coupling  ..?  FOR  WENCHED  & 

f  PR.  ROTARY.....  _ 

Fig.    243 — Ship  Tuning  Record    (American   Marconi   Company). 


in     accordance. 

radiated  wave 
may  therefore  possess 
a  damping  factor  as 
low  as  that  obtained 
with  the  inductively 
coupled  system. 

If  the  coupling  re- 
quires still  further  re- 
duction, contacts  C 
and  D  are  moved  up- 
ward and  away  from 
A,  B,  care  being  taken 
to  keep  the  same 
number  of  turns  be- 
tween C  and  D  in 
order  to  maintain  res- 
onance. 

Loose  coupling  can 
be  obtained  as  shown 
in  Fig.  242  where  the 
condenser  C  is  of 


such  capacity  that  to  obtain  resonance  with  the  antenna  circuit  only  one-half  turn  is  required 
between  A  and  B.  Hence,  the  mutual  inductance  between  the  open  and  closed  circuits  is  small 
and  the  radiated  waves  approach  unity. 

183.  Measurement  of  High  Voltages. — The  maximum  voltage  per  alter- 
nation of  a  high  voltage  current  can  be  obtained  by  the  sphere  gap  method. 
Two  brass  spark  balls,  two  centimeters  in  diameter,  are  mounted  on  an  insulating 
base  and  arranged  so  that  the  length  of  the  gap  can  be  increased  or  decreased  by 
a  micrometer  adjustment. 

The  gap  is  then  connected  to  a  given  source  of  high  voltage  current  and  the  gap  length- 
ened until  sparking  just  ceases.  For  a  given  separation  of  the  electrodes,  the  voltage  corre- 
sponding thereto  is  shown  in  the  following  table: 


Spark  Length  in 
Millimeters. 

1 

2 

3 

4 

5 

6 

7 

8 

9 
10 
11 
12 
14 


Voltage. 
4,700 
8,100 
11,400 
14,500 
17,500 
20,400 
23,250 
26,100 
28,800 
31,300 
33,300 
35,500 
38,700 


Spark  Length  in 
Millimeters. 
16 
18 
20 
22 
24 
26 
28 
30 
32 

'  34 
36 
38 
40 


Voltage. 
41,300 
44,700 
47,400 
49,800 
52,000 
54,000 
55,800 
57,500 
59,000 
60,400 
61,800 
63,000 
64,200 


184.  Tuning  and  Adjustment  Record. — After  a  radio  transmitter  of  any 
type  has  been  adjusted  by  a  Marconi  Inspector,  a  complete  tuning  record  is  posted 
in  the  operating  room.  A  typical  record  is  shown  in  Fig.  243,  which,  as  will  be 
observed,  indicates  the  number  of  turns  in  the  primary  and  secondary  windings 
of  the  oscillation  transformer,  also  in  the  loading  coils  for  each  of  the  three 
standard  wave  lengths.  In  the  event  of  repairs  being  made,  the  adjustments 
noted  on  this  record  must  positively  be  duplicated. 


PRACTICAL  RADIO  MEASUREMENTS. 


221 


In  counting  the  helix  turns  for  any  standard  wave,  the  operator  should  note 
first,  the  point  on  the  helix  at  which  the  circuit  begins  and  also  the  general  direc- 
tion of  the  turns  for  an  increase  of  inductance.  Pie  should  then  be  sure  to  count 
the  number  of  complete  turns  of  inductance  and  not  merely  the  bonds  on  the  helix. 


TURNS 


3  TURNS 


COUPLING 
|  TURN  COMMON 


COUPLING:  8  INCHES 
OR  IF  GRADUATED 
SCALE  GIVE  READING^ 
THUS:  COUPLING     i 

SCALE  14  ' 


Zj  TURNS 
COUNTING 
FROM 
OUTSIDE 


2  TURNS  - 
COUNTING  FROM 
OUTSIDE 


l{  TURNS 


—  G 


TURNS 


COUPLING  4 
INCHES  OR  IF 
GRADUATED  SCALE 
/'GIVE  READING  THUS! 
COUPLING  SCALF  14.5 


TURNS 


2{ TURNS 


COUPLING 
NO  COMMON 
TURNS 
BETWEEN  PAS 


l{  TURNS 


I  TURNS 


COUPLING     4  INCHES   OR 
IF  GRADUATED  SCALE, GIVE 
READING  THUS. 
COUPLING  SCALE    4.6 


—  G 


TURNS 


OR  IF  6RADUMID  COUPLING 
SCALE  IS  PROVIDED. INDICATE 
THUS  COUPLING  SCALE  45° 


Fig.  244 — The  Method  of  Checking  up  Coupling  and  Turns  of  a  Helix  as  Authorized 
by  the  U.  S.  Government. 

The  tuning  adjustments  of  the  quenched  spark  discharger  are  invariably  rather  critical 
and,  therefore,  the  settings  of  inductance  noted  on  the  tuning  cards  must  be  more  closely 
duplicated  than  for  sets  employing  simple  types  of  spark  dischargers.  A  slight  variation  of  the 
antenna  inductance  is  permissible  from  time  to  time  to  compensate  for  detuning  occasioned 
by  sagging  of  the  wires. 


222 


PRACTICAL  WIRELESS  TELEGRAPHY. 


The  coupling  reference  at  the  bottom  of  the  card  refers  to  the  coupling  of  the  primary 
and  secondary  windings  on  the  panel  sets  shown  by  the  scale  attached  to  the  coupling 
handle.  The  notation  "8  for  rotary"  means  that  the  coupling  for  the  rotary  gap  is  less  than 
for  the  quenched  gap  or  at  least  the  primary  and  secondary  windings  have  greater  separation 
than  for  title  quenched  gap. 

The  notation  condenser  jars  refers  to  the  number  of  jars  in  use  in  the  closed  circuit. 

MARCONI  WIRELESS  TELEGRAPH  COMPANY-  OF  AMERICA 

STATION       /»  £-   /   tH  L> 


SECONDARY    SCALE    SETTINGS. 


Fig.    .245— Curve    Showing    Reduction    of    Current    by    Turning    the    Secondary    Winding    of    the    Type    A 
Oscillation  Transformer  at  Right  Angles  to  the  Primary  Winding. 


Diagrams  showing  the  different  ways  Government  Inspectors  record  the  coupling  and 
the  number  of  primary  and  secondary  turns  for  the  standard  wave  lengths  are  shown  in 
Fig.  244  which  is  a  duplicate  of  the  drawings  printed  on  the  rear  of  the  Government  tuning 
record  (form  766).  Duplicates  of  these  sketches  are  drawn  in  on  the  Government  tuning 
record  by  the  Government  Inspectors  and  the  exact  number  of  turns  for  the  standard  waves 
in  either  circuit  plainly  marked  for  operator's  reference.  In  certain  standard  types  of 
Marconi  apparatus,  a  diagram  showing  the  turns  in  use  is  not  required;  the  inspector 
merely  marks  the  number  of  turns  for  the  standard  waves  in  both  the  primary  and  secondary 
windings. 

We  have  shown  in  paragraph  103  how  the  coupling  of  the  type  A  oscillation  transformer 
is  reduced  by  turning  the  secondary  winding  at  an  angle  to  the  primary.  It  is  also  mentioned 
in  paragraph  202  that  the  antenna  current  can  in  this  way  be  progressively  increased  from 
zero  to  maximum. 

In  the  curve  shown  in  Fig.  245  the  antenna  current  corresponding  to  various  angles  of  the 
secondary  winding  is  plotted  in  the  form  of  a  curve.  Such"  records  are  posted  in  the 
operating  room.  It  will  be  observed  in  Fig.  245  that  when  the  secondary  winding  bears  an 
angle  of  90°  to  the  primary  winding,  the  antenna  current  is  zero,  but,  as  the  secondary 
winding  is  turned  towards  maximum  coupling,  the  antenna  current  gradually  increases  to 
8J/2  amperes.  By  giving  careful  attention  to  the  data  shown  in  Fig.  245  the  operator  is 
enabled  to  communicate  with  ship  or  shore  stations  with  the  minimum  of  interference,  using 
such  values  of  antenna  current  only  as  will  enable  the  desired  distance  to  be  covered. 
There  is  no  hard  and  fast  rule  to  lay  down  for  determining  the  correct  value  of  antenna 
current  for  transmitting  over  a  given  distance,  but  after  a  few  experimental  trials  with 
various  stations,  the  minimum  current  for  a  given  range  is  easily  found  out. 


PART  XII. 

STANDARD  MARINE  SETS  OF  THE  AMER- 
ICAN MARCONI  COMPANY. 

PANEL  TRANSMITTERS— COMPOSITE  TRANSMITTERS. 

185.  PANEL  TRANSMITTERS.  186.  DETAILS  OF  TYPE  P-4 
PANEL.  187.  DESCRIPTION  OF  APPARATUS.  188.  COMPLETE 
ADJUSTMENT  OF  TYPE  P-4  SET.  189.  TYPE  P-5  PANEL  TRANS- 
MITTER. 190.  DESCRIPTION  OF  APPARATUS.  191.  COMPLETE 
ADJUSTMENT  OF  THE  TYPE  P-5  SET.  192.  How  TO  REMOVE 
THE  ARMATURE  OF  THE  l/2  K.  W.  MOTOR  GENERATOR.  193. 
THE  1  K.  W.  NON-SYNCHRONOUS  DISCHARGER  TRANSMITTER. 
194.  DESCRIPTION  OF  THE  SET.  195.  INSTALLATION.  196. 
ADJUSTMENT  OF  THE  1  K.  W.  SET.  197.  TYPE  "E-2"  ONE- 
HALF  KILOWATT,  120  CYCLE  PANEL  TRANSMITTER.  198. 
DETAILS  OF  THE  CIRCUITS  AND  APPARATUS,  199.  GENERAL 
INSTRUCTIONS  FOR  TUNING  AND  ADJUSTING.  200.  MARCONI 
2  K.  W.  240  CYCLE  TRANSMITTER.  201.  TYPE  P-9  y4  K.  W. 
CARGO  TRANSMITTING  SET.  202.  AERIAL  CURRENT  AND  REDUC- 
TION OF  POWER.  203.  GENERAL  INSTRUCTIONS  FOR  THE  PANEL 
SETS. 

185.  Panel  Transmitters. — In  point  of  efficiency,  general  utility,  economy 
of  space,  and  ease  of  installation,  the  panel  transmitter  units  of  the  Marconi 
Company  of  America  excel.  Three  types  are  now  supplied  for  marine  service 
known  as  Type  P-4,  P-5  and  P-9.  The  former  has  a  normal  power  consumption 
of  2  K.  W.',  the  latter  y2  K.  W. 

The  2  K.  W.  set  has  a  daylight  range  of  450  to  650  miles,  1,500  to  2,500  miles 
after  dark.  The  */>  K.  W.  set  has  a  daylight  range  varying  from  250  to  400 
miles,  600  to  1,500  miles  after  dark. 

Panel  transmitter  type  P-4  is  furnished  for  vessels  of  large  tonnage  requiring 
the  maximum  possible  transmitting  range,  but  type  P-5  is  particularly  suitable 
for  small  yachts,  cargo-carrying  vessels  and  tug  boats.  It  should  be  kept  in  mind, 
however,  that  set  P-5  has  sufficient  range  to  comply  with  the  international  regu- 
lations on  vessels  of  any  tonnage. 

Although  the  y2  K.  W.  and  2  K.  W.  panel  transmitters  arc  now  standard  for  ship  service, 
a  number  of  vessels  in  the  American  Marconi  Company's  service  are  still  fitted  with  com- 
posite sets  popularly  known  as  the  "1  K.  W.  non-synchronous  set"  and  the  "2  K.  W.  240 
cycle  set.''  Several  vessels  are  fitted  witli  a  J/2  K.  W.  120  cycle  panel  transmitter  (Type  E-2) 
but  the  manufacture  of  this  type  has  been  discontinued. 

Ill  this  chapter,  all  sets  now  in  use  in  the  ship  service  will  be  described  in  de- 
tail and  such  information  will  be  supplied  as  will  enable  the  operator  to  adjust  the 
apparatus  to  its  maximum  degree  of  efficiency.  The  2  K.  W.  panel  set  will  first 
be  described* 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO.  225 

186.  Details  of  Type  P-4  Panel. — The  complete  2  K.  W.  panel  set  comprises 
the  following  apparatus: 

(1)  Panel  Transmitter  consisting  of  the  necessary  power  measuring  instru- 
ments,  oscillation   transformer,  condenser,  variable  and  plug  type  aerial 
tuning     inductance,    quenched    spark    gap,    motor    and    generator    field 
rheostats,   wave   length   changing   switch,   several  resistance  units  and  a 
compensating  reactance  regulator. 

(2)  Crocker- Wheeler  or  General  Electric  2  K.  W.  500  Cycle  Motor  Generator 
with  synchronous  rotary  spark  gap  mounted  on  the  generator  shaft,  pro- 
tective condensers,  starting  resistance,  automatic  motor  starter  and  neces- 
sary controlling  appliances. 

(3)  Type    106    Receiving    Tuner    with    crystal    rectifier    and    head    telephone 
receiver. 

(4)  Type  I  Aerial  Change-Over  Switch  with  necessary  appliances  for  protect- 
ing the  receiving  apparatus  from  the  transmitter. 

(5)  Type  C  Transmitting  Key. 

(6)  High  Potential  Closed  Core  Transformer. 

In  addition  to  the  foregoing  apparatus,  the  owners  of  the  vessel  supply  a  60  cell  storage 
battery  and  a  charging  panel  fitted  with  the  necessary  appliances-  for  controlling  the  charging 
circuits.  With  an  aerial  equipment  like  that  described  in  paragraph  117,  the  foregoing  ap- 
paratus completes  a  standard  marine  wireless  telegraph  equipment. 

A  simple  fundamental  diagram  of  the  complete  circuits  of  the  type  P-4  transmitter 
and  receiver  is  shown  in  Fig.  246,  a  front  view  of  the  panel  in  Fig.  247,  a  side  view  in  Fig. 
248  and  a  rear  view  in  Fig.  249. 

187.  Description  of  Apparatus. — The  motor  generator  consists  of  a  4  H.  P. 
110  volt  2  pole  direct  current  motor  connected  directly  to  a.  2  K.  W.  500  cycle  alternator. 
The  motor  is  specially  constructed  to  operate  with  little  variation  of  speed  on  pressures 
varying  from  95  to  115  volts. 

The  generator  is  of  the  rotating  armature  type,  having  a  normal  open  circuit  voltage  of 
350  volts,  and  a  working  or  load  voltage  of  140  volts.  The  voltage  of  the  generator  is  varied 
by  a  rheostat  of  the  sliding  contact  type  mounted  on  the  right  hand  side  of  the  panel,  and 
the  speed  of  the  motor  by  a  similar  rheostat  on  the  left  hand  side  of  the  panel.  (See 
Fig-  247.)  |  !*« 

An  automatic  motor  starter  is  provided  which  permits  the  motor  to  be  controlled  from  a 
distant  point  in  the  operating  room.  The  starter  is  of  the  magnetic  plunger  type  having  a 
piston  which  travels  through  a  cylinder  compressing  the  air  on  one  side  and  cre- 
ating a  vacuum  on  the  other.  The  speed  of  the  armature  accelerates  uniformly  and  the 
time  of  completing  the  circuit  may  be  varied  by  a  special  adjustment  screw  attached  to  the 
starter. 

The  starter  is  fitted  with  an  clcctrodynamic  brake  which  comprises  a  resistance  coil 
thrown  in  shunt  to  the  armature  when  the  current  to  the  motor  is  turned  off.  By  means 
of  this  brake,  the  motor  is  brought  to  a  standstill  within  ten  seconds. 

An  overload  relay  switch  connected  in  series  with  the  armature  automatically  opens  the 
D.  C.  circuit  to  the  starter  solenoid  when  current  in  excess  of  a  certain  number  of  amperes 
passes  through  the  motor  windings.  In  case  of  overload,  the  plunger  of  the  starter  drops 
down  breaking  the  main  circuit  to  the  motor  armature.  The  circuit  of  the  solenoid  then 
remains  open  until  the  main  D.  C.  line  switch  is  opened  by  hand.  If  a  short  circuit  exists 
in  any  part  of  the  wiring  and  the  power  switch  is  closed  again,  the  relay  will  open  the 
circuit  once  more  and  continue  to  do  so  until  the  trouble  is  located.  The  relay  is  generally 
adjusted  to  open  at  35  amperes. 

Each  terminal  post  of  the  motor  generator  is  connected  to  one  terminal  of  a  protective 
condenser  to  neutralize  differences  of  potential  that  may  be  set  up  by  electrostatic  induction 
from  the  transmitting  apparatus.  The  opposite  terminal  of  the  condenser  is  connected  to 
the  frame  of  the  motor  generator  and  then  to  earth  (see  Fig.  79).  Six  protective  condensers 
are  provided  for  each  motor  generator,  and  enclosed  in  a  metallic  case.  The  frame  of  the 
motor  generator  and  also  the  lead  covered  wires  connecting  to  the  apparatus  on  the  panel 
are  thoroughly  connected  to  earth. 


226 


PRACTICAL   WIRELESS   TELEGRAPHY. 


AERIAL 
AMMETER 


MOTOR 
RHEOSTAT 


COUPLING 
ADJUSTED 


WATTMETER 


GENERATOR 
.-RHE05TAT 


AERIAL  TUNING 
~~  INDUCTANCE 


-WAVE  LENGTH 
CHANGING  SWITCH 


MOTOR 
GENERATOR 


QUENCHED 
i-—    GAP 


OVERLOAD 
RELAY 


AUTOMATIC 
"STARTER  "oe"' 


ROTARY 
GAP 


Fig.   247— Front   View   Marconi   2    K.   W.    500   Cycle   Transmitting   Set. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 
-SLIDING  CONTACT 

CONTINOUSLY    VARIABLE 

TUNING  INDUCTANCE 

PLUG    AER\AL    TUNING 
!NDUCTA!^4CE 


227 


SECONDARY 
/OSCILLATION 
TRANSFORMER 


PRIM.WAVE    LENGTH  SWITCH 


SPARK  GAP  CHANGING  SWITCH 


STATIONARY  ELECTRODES 
ROTARY  GAP 


WAVE.  LENGTH 
SECONDARY  SWITCH 


MOTOR 
'STARTER 
RESISTANCE 


STEP  UP  TRANSFORMER 
Fig.    248— Side    View    Marconi    2    K.    W.    500    Cycle    Transmitting    Set. 


228 


PRACTICAL   WIRELESS  •  TELEGRAPHY. 


AERIAL  TUNING    INDUCTANCE 


SEC.  WAVE  LEN&TH 
CHANGING  SWITCH 


The  circuit  from  the  alternator  to  the  transformer  includes  a  direct  reading  wattmeter 
for  indicating  the  power  consumed  by  the  transformer.  The  current  coil  of  this  instrument 
is  connected  in  series  with  the  transformer  primary  and  the  potential  coil  is  connected  across 
its  terminals. 

The   high  voltage   transformer,  of  the   closed  core  type,   is   immersed   in   a   semi-liquid 

grease.  The  primary  wind- 
ing is  connected  to  the  con- 
trol panel  by  means  of  lead- 
covered  wires  which  have 
their  covering  grounded  to 
the  transformer  case  and  to 
the  panel  frame.  The  sec- 
ondary terminals  of  the 
transformer  are  brought  out 
through  two  insulators  and 
are  mounted  thereon.  A 
protective  spark  gap  is  pro- 
vided which  permits  a  dis- 
charge in  case  the  voltage 
exceeds  a  certain  critical 
value.  The  secondary  po- 
tential is  approximately  12,- 
500  volts.  The  secondary 
terminals  of  the  transformer 
are  connected  directly  to  the 
terminals  of  the  high  poten- 
tial condenser  of  the  closed 
oscillation  circuit.  Gener- 
ally, the  transformer  is 
screwed  to  the  floor  imme- 
diately to  the  rear  of  the 
control  panel. 

The  transmitting  con- 
denser comprises  six  copper- 
plated  glass  jars  of  .002 
microfarads  capacity  each, 
three  of  which  are  connect- 
ed in  parallel  for  the  300 
meter  wave,  and  six  in 
parallel  for  the  450  and  600 
meter  wave.  These  are 
mounted  in  a  metal  rack, 
placed  directly  underneath 
the  oscillation  transformer 
and  connected  thereto  with 
a  multipoint  switch  to  be 
explained  later. 


MOTOR  GENERATOR 


MOTOR  STARTER 
RESISTANCE 


ROTARY 

SPARK  GAP          \1 


TRANSFORMER 


Fig.    249— Rear   View   Marconi  2  K.    W.    500  Cycle  Transmitting   Set. 


The      oscillation     trans- 


former is  of  the  inductively 
coupled  type,  the  primary  and  secondary  windings  consisting  of  a  strip-copper,  spiral  wound 
edgewise  on  a  rectangular  insulating  support.  The  secondary  turns  for  the  three  standard 
waves  are  selected  by  means  of  three  flexible  plug  connections,  but  the  three  taps  on  the 
primary  inductance  for  the  300,  450  and  600  meter  waves  are  soldered  fast  in  position, 
connections  being  shifted  from  one  wave  to  the  other  by  means  of  a  multipoint  wave-length 
changing  switch. 

Two  aerial  tuning  inductances  are  provided,  one  being  a  continuously  variable  inductance 
having  a  sliding  contact  which  can  be  revolved  by  a  handle  in  the  front  of  the  panel,  per- 
mitting the  inductance  to  be  increased  or  decreased  inch  by  inch.  The  second  aerial  tuning 
inductance,  connected  in  series  with  this  one,  is  fitted  with  three  plug  connections  which  are 
placed  at  positions  on  the  coils  corresponding  to  the  300,  450  and  600  meter  waves. 

The  short  wave  condenser,  which  consists  of  2,  3  or  4  copper-plated  jars  connected  in 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


229 


series,  mounted  in  a  metallic  rack,  is  shown  connected  in  the  aerial  circuit  in  Fig.  246.    When 
not  in  use,  the  jars  are  shunted  by  a  flexible  conductor  or  by  a  switch. 

The  aerial  ammeter  is  a  direct  current  instrument,  range  0-20  amperes,  its  terminals  being 
connected  to  a  thermo-couple  which  is  in  turn  mounted  on  a  heating  wire,  the  latter  being 
connected  directly  in  series  with  the  antenna  system.  (See  Fig.  53.) 

The  Type  I  Aerial  Change-Over  Switch  is 
fitted  with  the  necessary  contacts  for  opening 
and  closing  the  transformer  primary  circuit 
and  for  protecting  the  receiving  tuner  circuits 
during  the  periods  of  transmission.  It  also  discon- 
nects the  secondary  winding  of  the  oscillating  trans- 
former from  the  transmitter  aerial  during  the  re- 
ceiving period,  interrupts  the  field  circuit  to  the  gen- 
erator, and  starts  and  stops  the  motor  generator. 
This  switch  is  described  in  detail  in  paragraph  155. 

The  predominant  feature  of  the  2  K.  W.  500  cycle 
set  is  the  specially  designed  wave-length  changing 
switch,  the  function  of  which  will  be  fully  described 
in  paragraph  188,  section  d.  This  switch  auto- 
matically shifts  the  coupling,  inductance  and  capacity 
values  for  the  three  standard  waves,  the  entire  opera- 
tion being  performed  by  merely  throwing  a  handle 
mounted  on  the  front  of  a  panel. 

To  meet  the  government  requirements,  provision 
is  made  for  transmitting  at  low  power.  A  fixed  re- 
sistance connected  in  series  with  the  generator  field 
permits  the  voltage  of  the  generator  to  be  reduced  to 
the  lowest  operative  condition.  The  switch  marked 
"low  power"  shunts  this  resistance  coil,  and,  when 
open,  only  one  or  two  gaps  of  the  quenched  dis- 
charger are  to  be  employed. 

Type  P-4  set  is  equipped  with  a  synchronous 
rotary  gap,  having  30  sparking  points,  mounted  on  the 
shaft  of  the  motor  generator,  the  disc  being  thor- 
oughly connected  to  earth ;  also  a  quenched  spark  gap 
consisting  of  15  plates  mounted  in  a  metal  rack  and 
insulated  therefrom.  The  disc  of  the  rotary  gap  is 
fitted  with  air  circulating  vanes  which  force  air  at 
uniform  pressure  through  a  specially  designed  air 
duct  to  the  plates  of  the  quenched  gap. 

188.  Complete  Adjustment  of  Type  P-4  Set.— (a)  Plunger  Control  Auto- 
matic Starter.  The  plunger  of  the  automatic  motor  starter  should  be  adjusted  to  reach  the  full 
running  position  within  12  seconds.  A  small  regulating  screw  P,  Fig.  251,  is  placed  directly 
underneath  the  plunger  chamber  and,  if  turned  to  the  right,  the  plunger  moves  up  slowly, 
but  to  the  left,  rapidly.  The  contacts  F,  G,  H,  I,  J,  which  make  connection  with  the  crossbar 
on  the  plunger  K,  should  be  adjusted  to  complete  the  circuit  to  the  armature  in  progression; 
e.  g.,  one  is  placed  a  little  bit  higher  than  the  other  as  shown. 

Set  screw  U  should  be  adjusted  until  lever  O  of  the  overload  relay  is  opposite  30  or  35 
amperes.  This  is  approximately  the  correct  setting  for  average  working.  The  tension  on  the 
springs  attached  to  the  upper  contacts  of  the  automatic  starter,  should  be  adjusted  for  firm 
contact  with  bar  K. 

The  circuit  to  the  solenoid  N  is  opened  and  closed  at  T-l  while  contact  T  closes  the  cir- 
cuit to  a  special  magnet  winding  holding  lever  O  upward  in  case  of  short  circuit  in  the 
power  circuits.  Contacts  Y  and  Z  throw  a  small  resistance  coil  in  series  with  the  solenoid 
winding  after  the  starter  has  reached  the  full  running  position.  A  simple  fundamental 
wiring  diagram  of  this  starter  appears  in  Fig.  75. 

(fc)  Adjustment  of  the  Spark  Note.  In  the  adjustment  of  the  note  of  the  quenched  spark 
discharger,  it  is  to  be  understood  that  the  pitch  depends  upon  conditions  of  resonance 
between  the  open  and  closed  oscillation  circuits  as  well  as  upon  the  voltage  of  the  generator. 
Careful  regulation  of  the  voltage,  however,  is  the  principal  adjustment  and  the  one  to  be 
undertaken  first  (provided  the  set  has  been  tuned). 


Type 

Panel     Transmitter     (American     Mar- 
coni   Company). 


230 


PRACTICAL   WIRELESS   TELEGRAPHY. 


The  operator  should  select  a  certain  number  of  gaps,  say  eight,  and  follow  it  by  varying 
the  generator  voltage.  After  the  set  has  been  tuned  for  maximum  antenna  current,  the 
voltage  should  be  slightly  readjusted  until  the  spark  is  clear. 

It  is  easily  seen  that  if  the  note  is  clear  and  the  secondary  circuit  is  thrown  out  of 
resonance,  less  energy  will  'be  withdrawn  from  the  closed  oscillation  circuit,  which  will 
increase  the  voltage  across  the  gap.  The  pitch  of  the  note  will  therefore  be  destroyed. 

The  rule  to  be  followed  is,  tune  the  set  first;  afterwards  adjust  the  gaps  and  voltage  until 
the  note  is  clear  and  the  wattmeter  indkates  2  kilowatts. 

The  synchronous  rotary  gap  is  adjusted  for  a  high  pitched  note  by  the  small  brass  rod 
R  and  knob,  Fig.  116,  which  moves  the  muffling  drum  carrying  the  stationary  spark  elec- 
trodes through  a  25-degree  arc. 

It  is  also  essential  that  the  A.  C.  voltage  be  carefully  regulated.  But  the  operator  should 
take  care  that  the  reading  of  the  wattmeter  in  no  case  exceeds  2  K.  W.  Also  he  should 
adjust  the  length  of  the  discharge  gap  until  the  stationary  electrodes  and  the  rotary  elec- 
trodes are  separated  no  more  than  .01  of  an  inch.  The  complete  process  will  perhaps  be 
clearer  from  the  detailed  drawing,  Fig.  252. 


OVERLOAD  REUW 

o  o  o   o 


o 

P-6 


O 

P6 


O 

PIO 


o 

p-ll 


o 

P12 


o 

PIS 


o 

PI4 


O 

P-15 


Fig.    251 — Automatic    Starter    Panel    (2    K.    W.     500    Cycle    Transmitter). 


It  is  important  that  this  adjustment  be  not  undertaken  until  the  motor  is  stopped.  Then 
one  stationary  electrode  is  lowered  down  by  means  of  the  milled  screw  S  until  it  touches  one 
of  the  electrodes  on  the  disc.  The  electrode  is  then  raised  to  just  miss  the  rotating  electrodes 
vvhen  the  disc  is  turned  over  by  hand.  Similarly  the  other  stationary  electrode  must  be 
lowered  until  it  touches  the  disc  electrodes  and  then  raised  until  it  clears  the  entire  set  of 
spark  points  on  the  disc. 

Each  turn  of  the  nut  S  represents  a  radical  movement  of  .037  of  an  inch  and  as  there  are 
eight  holes  equally  spaced  in  the  face  of  the  nut  (which  engage  the  pin  P),  the  movement 
of  one  hole  represents  a  radial  movement  of  approximately  .005  of  an  inch.  The  length  of 
the  spark  discharge  gap  is  adjusted  correctly  when  the  electrodes  on  the  disc  and  the  sta- 
tionary electrodes  are  separated  .005  of  an  inch  to  .01  of  an  inch,  and,  after  they  have  been 
so  adjusted,  the  muffling  drum  should  be  shifted  backward  or  forward  until  a  uniform 
clear  spark  note  is  obtained. 

(c)  Details  of  Quenched  Gap.  A  detailed  front  view  of  this  spark  gap  partly  in  section 
is  shown  in  Fig.  253.  The  quenched  gap  consists  of  a  number  of  copper  discs  or  plates  such 
as  O,  O,  having  circular  sparking  surfaces  J  separated  by  paper  washers  which  are  specially 
treated  with  an  insulating  varnish.  These  plates  are  set  in  a  trough  and  clamped  together 
with  such  pressure  that  the  space  enclosed  between  the  discs  is  airtight.  A  disc  or  insulating 
material  F  is  placed  against  the  end  plates  to  insulate  the  gap  from  brackets  K  and  C.  Con- 
tacts P  and  Q,  which  extend  to  the  condenser  and  oscillation  transformer,  may  be  plugged 
into  the  cooling  flanges  of  any  of  the  plates.  Nuts  D,  D  brace  the  end  castings  K  and  C. 

Operators  of  the  Marconi  service  should  give  the  following  instructions  careful  consider- 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


231 


INSULATING 
/BUSHING 


ation :  In  setting  up  the  quenched  gap  care  should  be  taken  to  keep  the  spark  surfaces  J 
absolutely  clean  and  smooth.  The  paper  discs  or  washers  N  should  also  be  scrupulously 
clean.  Fifteen  plates  are  stacked  up  in  the  rack  and  pressure  applied  through  the  pressure 
_  bolt  B,  which  is  locked  in  position  by  the 

nut  A.    This  screw  should  be  taken  up  with 
a  10-inch  monkey  wrench. 

If  the  gap  is  not  airtight  the  spark  note 
will  be  irregular.  This  is  a  positive  indica- 
tion that  the  gap  is  leaking  and  it  should 
therefore  be  taken  apart  and  the  sparking 
surfaces  cleaned  and  reassembled,  using  new 
gaskets  if  necessary.  The  gap  should  not  be 
used  over  an  extended  period  if  it  once  starts 
leaking. 

In  case  one  of  the  gaps  become  defective 
it  can  be  short-circuited  by  means  of  a  metal 
contactor  which  is  furnished  especially  for 
that  purpose.  The  defective  gap  can  be  de- 
tected by  means  of  a  short  circuiting  contact 
mounted  on  an  insulating  rod  called  a  test 
rod.  The  gaps  can  be  successively  short- 
circuited  and  the  defective  one  will  be  located 
by  there  being  no  sparking  at  the  tips  of  the 
contactor. 

In  event  the  gap  is .  taken  apart,  before 
being  reassembled,  the  sparking  surfaces  of 
the  copper  plates  must  be  cleaned  and 
smoothed  off  by  means  of  very  fine  sand- 
paper. Care  should  be  taken  to  keep  the  sur- 
faces perfectly  flat  during  the  cleaning  so 
that  the  opposing  surfaces  will  be  parallel 
when  they  are  placed  in  the  rack.  When  the 
gap  is  in  proper  working  condition,  the 
sparking  surfaces  show  a  uniform  coloring 
over  the  whole  area.  //  the  surfaces  of  the 
plate  are  black  or  dark  colored,  it  is  an  indi- 
cation that  the  gap  is  not  airtight. 

If  it  becomes  necessary  to  disassemble  the 
gap,  after  reassembly,  the  spark  must  be  dis- 
charged through  the  series  of  plates  con- 
tinually for  one  or  two  hours.  In  fact,  the 
note  will  not  be  clear  until  the  gap  is  thus 
"seasoned." 

(d)  Tuning  and  Wave-Length 
Changing  Switch.    It  has  long  been  the 
desire  of  radio  engineers  to  fit  the  trans- 
Fig-  252™Showi"! HT  the  st*tionary  S^ctr,°3es  of  mitting  apparatus  with  appliances 

the  Marconi   Synchronous  Gap  are  Adjusted.  111  1  i         r     i 

whereby  the  wave-length  of  the  trans- 
mitting set  could  be  quickly  changed  by  merely  throwing  a  switch,  but  the  re- 
quirements of  radio-frequency  circuits  would  not  permit  this  to  be  done  without 
certain  mechanical  and  electrical  considerations. 

For  example :  With  the  potentials  employed  in  radio-telegraph  transmitters,  the  switch 
to  perform  these  functions  must  possess  first-rate  insulating  qualities.  The  associated  circuits 
and  leads  thereto  must  be  widely  separated  and  in  addition  they  must  not  be  of  too  great 
length  in  order  that  the  length  of  the  radiated  wave  may  be  kept  within  restrictions.  Again 
the  component  parts  of  the  complete  switch  cannot  be  placed  in  any  convenient  mechanical 
position  but  must  perform  their  functions  near  to  the  portion  of  the  circuit  in  conjunction 
with  which  they  operate.  Additional  considerations  are  involved  obvious  to  the  radio 
engineer,  but  not  of  sufficient  importance  to  be  gone  over  in  detail. 

The  complete  process  of  tuning  of  the  2  K.  W.  500  cycle  sot,  and  the  locating  of  the  taps 


STATIONARY 
ELECTRODE 


ELECTRODE 
OH  DISC 


232 


PRACTICAL  WIRELESS  TELEGRAPHY. 


on  the  secondary  turns  of  the  oscillation  transformer  may  appear  difficult  to  the  unskilled 
operator,  but  is  easily  accomplished  by  the  trained  engineer  or  inspector. 

The  function  of  the  wave  length  changing  switch  and  the  tuning  of  the  2  K.  W.  500  cycle 
get  will  be  described  in  detail  and  reference  should  be  made  particularly  to  Fig.  246  showing 
the  complete  circuits  of  the  apparatus. 

First  it  should  be  noted  that  when  the  quenched  spark  discharger  is  cut  in  the  circuit,  the 
coupling  betiveen  the  primary  and  secondary  windings  is  varied  by  varying  the  number  of 
turns  in  the  secondary  winding  only,  the  primary  and  secondary  coils  being  placed  in  a 
fixed  mechanical  position,  but  for  the  synchronous  rotary  spark  gap  they  are  drazvn  apart  to 
whatever  degree  of  separation  is  necessary  to  radiate  a  pure  wave.  In  certain  installations 
the  primary  and  secondary  windings  may  be  placed  in  a  fixed  mechanical  position  for  the 
rotary  discharger  as  well  as  for  the  quenched  gap. 

(1)  Explanation  of  the  circuits.  It  will  be. observed  from  the  diagram,  Fig.  246,  that 
these  sets  are  fitted  with  a  change-over  switch  which  permits  either  the  rotary  disc  discharger 
or  the  quenched  spark  discharger  to  be  cut  in  the  circuit.  The  gaps  are  connected  in  series 
and  to  change  from  one  to  the  other,  it  is  only  necessary  to  shift  the  position  of  the  switch 
K.  When  the  quenched  discharger  is  in  use,  a  compensating  inductance  L,  Fig.  246,  makes 
up  for  the  length  of  leads  connected  from  contacts  58  and  59  to  the  rotary  disc  discharger, 
thereby  keeping  the  wave-length  of  the  closed  oscillatory  circuit  constant.  When  the  rotary 


D     _*s 


Fig.    253— Detailed    Sketch    of    Marconi    Quenched    Spark    Gap. 

discharger  is  connected  in  the  circuit,  the  flexible  leads  attached  to  the  quenched  gap  plates 
must  be  connected  together. 

In  the  diagram,  Fig.  246,  the  high  potential  transformer  of  the  set  is  represented  by  the 
primary  and  secondary  windings  P  and  S  respectively;  the  high  potential  condensers  at  C-l 
and  C-2;  the  primary  winding  of  the  oscillation  transformer  at  P-l,  the  secondary  at  S-l ; 
also  an  aerial  tuning  inductance  S-2  with  flexible  contacts  A,  B  and  C,  and,  in  addition,  a 
continuously  variable  tuning  inductance  S-3.  The  antenna  system  further  includes  the  series 
condenser  C-3  and  the  aerial  ammeter.  The  distinctive  feature  of  this  apparatus  is  the 
primary  wave-length  changing  switch  W-l  operated  simultaneously  by  a  single  handle  in 
conjunction  with  the  switch  W-2.  It  will  be  observed  that  when  W-l  is  in  contact  with 
point  39,  a  single  unit  of  the  condenser  C-2  (three  jars  in  parallel  of  0.002  microfarads  each) 
is  connected  in  series  with  the  primary  winding  of  the  oscillation  transformer  at  point  A, 
i.e.,  the  adjustment  for  300  meters. 

Similarly,  W-2  closes  the  aerial  circuit  to  contacts  44  and  45,  which  connect  in  the  correct 
number  of  turns  to  radiate  the  300  meter  wave.  Contacts  A,  B  and  C  of  S-l  and  S-2  may 
be  plugged  in  at  any  point  on  the  secondary  coil,  but  they  are  permanently  attached  to  the 
coil  after  the  set  has  been  tuned  to  the  three  standard  waves.  Further  movement  of  the 
handle  connects  together  contacts  40  and  41,  which  not  only  connects  in  the  correct  number 
of  primary  turns  for  the  wave-length  of  450  meters,  but  also  connects  the  condenser  units 
C-l  and  C-2  in  parallel,  giving  a  total  condenser  capacity  of  0.012  microfarads.  Still  further 
movement  of  the  switch  closes  contacts  42  and  43,  also  contacts  48  and  49,  which  connect  in 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO.  233 

the  correct  number  of  turns,  in  both  the  open  and  closed  oscillatory  circuits,  for  the  standard 
600-meter  wave. 

(2)  The  Process  of  Tuning.  Before  undertaking  to  tune  a  set  of  this  type,  the  student 
should  thoroughly  consider  the  following  facts: 

(1)  In   order   to   obtain   the   maximum  flow  of  antenna  current,   a   different 
degree  of  coupling  at  the  oscillation  transformer  must  be  found  for  each 
of  the  standard  waves. 

(2)  In  order  that  the  radiated  waves  can  be  rapidly  changed  by  simply  throw- 
ing a  switch,  the  primary  and  secondary  windings  of  the  oscillation  trans- 
former must  remain  in  a  fixed  position  mechanically. 

(3)  The    coupling    of    the    oscillation    transformer    is    then    varied    for    each 
standard  wave   by   changing   the   self-inductance    of   the   secondary,   i.   e. 
cutting  in  or  out  turns. 

Now  the  correct  number  of  turns  for  the  secondary  winding  is  determined  for  each  of 
the  standard  waves  experimentally,  as  follows : 

For  preliminary  determination  at  a  standard  wave-length,  say  600  meters,  a  trial  number 
of  turns  (4  to  6  turns)  is  selected  at  the  secondary  winding  S-l  through  the  flexible  plug 
contact  C.  This  having  been  done,  a  second  trial  number  of  turns,  say  4  turns,  are  cut  in 
at  S-2.  Turns  are  added  or  subtracted  at  the  aerial  tuning  coil  S-3  (with  the  spark  gap 
discharging)  until  the  aerial  ammeter  indicates  a  maximum  deflection.  The  adjustment 
for  resonance  having  thus  been  located,  the  windings  P-l  and  S-l  are  drawn  apart  or  placed 
closer  to  ascertain  if  an  increase  of  antenna  current  will  result.  If  separation  of  P-l  and 
S-l  increases  the  antenna  current,  it  indicates  that  too  many  turns  have  been  included  at  S-l 
for  the  mechanical  position  of  the  coupling  coils  selected  at  the  start;  and,  in  consequence, 
turns  must  be  taken  out  at  S-l  and  additional  turns  cut  in  at  S-2  or  S-3,  until  resonance  is 
secured.  The  primary  winding  P-l  must,  however,  first  be  placed  in  its  original  fixed  posi- 
tion relative  to  S-l.  The  correct  number  of  turns  must  now  be  found  out  for  the  standard 
waves  of  450  and  300  meters,  that  is,  the  correct  number  of  turns  must  be  selected  and  the 
coupling  adjusted  until  the  maximum  antenna  current  for  each  wave  is  secured  with  P-l 
remaining  in  a  certain  fixed  position  to  S-l.  At  first  sight,  it  may  seem  difficult  to  locate 
the  correct  position  for  contacts,  A,  B  and  C,  on  S-l  and  S-2,  but  from  the  experience 
obtained  by  tuning  several  sets,  approximately  correct  inductance  values  may  be  selected  at 
sight.  It  should  be  understood  that  the  positions  of  A,  B  and  C  on  the  primary  and 
secondary  coils  shown  in  Fig.  246  do  not  necessarily  represent  their  relative  positions  in 
actual  commercial  practice.  With  aerials  of  different  capacity,  inductance,  and  resistance, 
decidedly  different  positions  must  be  located.  For  example:  At  the  wave-lengths  of  450 
and  600  meters,  less  inductance  may  be  required  at  point  C  than  at  point  B  for  these  two 
wave-lengths,  or  vice  versa. 

Careful  consideration  of  the  foregoing  procedure  will  reveal  that  (1)  the  continuously 
variable  inductance  S-3  aids  in  locating  the  number  of  turns  to  be  finally  connected  in  at 
S-2;  (2)  S-3  gives  a  fineness  and  quickness  of  adjustment  which  the  coil  S-2  with  a  flexible 
contact  does  not  afford;  (3)  the  mechanical  variation  of  the  coupling  between  P-l  and  S-l 
permits  the  correct  number  of  turns  to  be  selected  much  more  quickly  than  by  experimental 
trials  with  the  plug  contacts. 

It  is  now  clear  that  after  the  set  has  been  completely  tuned,  in  order  to  change  from  one 
standard  wave-length  to  the  other,  the  operator  needs  only  to  shift  the  handle  of  the  switches 
W-\  and  W-2,  with  the  exception  that  on  the  300-meter  wave,  the  short  wave  condenser  C-3 
must  be  connected  in  series  with  the  aerial  system.  In  certain  ship  installations,  the  eon- 
denser  C-3  is  connected  in  series  with  the  lead  from  contact  45  to  the  aerial  tuning  induct- 
ance S-2  and  when  so  connected  is  automatically  thrown  in  series  with  the  antenna  system 
whenever  the  switch  blades  W-l  and  W-2  are  shifted  to  the  300-meter  position. 

Because  of  the  perfect  quenching  secured  with  this  particular  type  of  gap  P-l  may  remain 
in  a  fixed  mechanical  position  relative  to  S-l  throughout  the  series  of  wave-lengths,  but 
when  the  rotary  disc  discharger  is  employed,  unless  a  complete  new  set  of  positions  is 
located  for  contacts  A,  B  and  C  of  S-l  and  S-2,  P-l  must  be  drawn  away  from  S-l  to 
secure  a  pure  wave.  P-l  might  have  to  be  placed  from  eight  to  ten  inches  away  from  S-l 
to  radiate  a  pure  wave  with  the  rotary  gap. 

The  primary  and  secondary  wave-length  changing  switch  performs  another  function 
which  has  not  been  mentioned :  When  the  switch  is  in  the  300-meter  position,  a  reactance 
foil  is  automatically  connected  in  series  with  the  primary  winding  of  the  power  transformer 


234 


PRACTICAL   WIRELESS   TELEGRAPHY. 


TERMINAL 


to  reduce  the  power  to  a  value  commensurate  with  the  decreased  secondary  condenser  capacity. 
The  transformer  then  consumes  1  k.  w. 

During  the  tuning  of  these  sets  it  has  been  observed  that  if  initial  adjustments  are  made 
near  to  metallic  dock  buildings,  the  effective  antenna  resistance  is  altered,  and,  in  conse- 
quence, the  tuning  adjustments  for  maximum  antenna  current,  need  to  be  changed  slightly 
zvhen  the  ship  is  at  sea.  Generally  it  is  only  necessary  to  vary  slightly  the  inductance  of  S-3 
for  maximum  aerial  current. 

After  the  set  has  been  tuned  in  this  manner,  the  purity  of  the  wave  and  decrement  of 
the  oscillations  is  measured  by  means  of  a  wavemeter  with  a  current  indicating  instrument 
connected  in  series,  such  as  a  wattmeter. 

The  commercial  value  of  a  transmitting  set  equipped  with  these  features  cannot  be  over- 
estimated. The  operator  having  at  his  disposal  three  standard  wave-lengths,  any  of  which 

may  be  brought  instantly  into  play,  can 
literally  pick  his  way  through  the  con- 
gested radio  atmosphere,  changing  from 
one  wave-length  to  the  other  as  operating 
conditions  may  require. 

O)  Type  106  Tuner.  The  working  and 
adjustment  of  this  tuner  is  completely  cov- 
ered in  paragraph  138  of  Part  IX. 

(/)  Short  Wave  Condenser.  This  con- 
denser consists  of  2,  3  or  4  jars  connected 
in  series.  Owing  to  the  variation  in  the 
capacity  of  commercial  aerials,  the  correct 
capacity  for  the  short  wave  condenser  is 
best  determined  by  experiment.  Two 
standard  jars  in  series  have  capacity  of 
.001  mfds.;  three  in  series  .00066  mfds.,  4 
in  series  .0005  mfds.  Aerials  having  a 
natural  wave-length  of  250  meters  do  not 
require  a  short  wave  condenser  for  three 


JAR 


254— One 


Type      of     Marconi 
Condenser. 


Short      Wave 


standard  waves.  A  side  view  of  one  type 
of  short  wave  condenser  is  shown  in 
Fig.  254. 

(g)  Spark  Gap  Change-Over  Switch. 
To  change  from  the  quenched  to  the 
rotary  gap,  a  small  D.  P.  D.  T.  switch  is 
connected  in  the  circuits  of  radio-fre- 
quency. When  the  rotary  gap  is  connected 
in  the  circuit,  the  quenched  gap  is  placed 
on  short-circuit  by  connecting  together  the 
contact  clips. 


189.  Type  P-5  Panel  Transmitter. — In  many  respects  this  set  duplicates 
the  type  P-4,  but  there  are  certain  mechanical  modifications.     The  motor  gen- 
erator and  the  transformer  are  mounted  on  the  panel  frame  together  with  the 
remainder  of  the  equipment.     Both  the  rotary  synchronous  discharger  and  the 
quenched  discharger  are  supplied.     A  compensating  inductance   joined  to  the 
change-over  switch  permits  the  use  of  either  spark  discharger  without  change 
of  the  wave-length  of  the  closed  oscillation  circuit. 

A  fundamental  circuit  diagram  of  the  automatic  starter  for  this  set  has  been  shown  in 
Fig.  74  and  a  more  detailed  diagram  is  now  presented  in  Fig.  255.  The  circuits  of  radio- 
frequency  are  shown  in  Fig.  256;  a  front  and  side  view  of  the  panel  in  Fig.  257  and  Fig. 
258  and  a  rear  view  in  Fig.  259. 

190.  Description  of  Apparatus. — The  motor  generator  comprises  a  110  volt 
direct  current  motor  directly  connected  to  a  y2  K.  W.  120  volt  500  cycle  alternator. 

The  motor  is  designed  to  operate  on  pressures  between  95  and  115  volts  with  little  change 
of  speed.  The  initial  speed  is  controlled  by  a  rheostat  mounted  on  the  left  hand  side  of 
the  panel. 

The  generator  is  of  the  inductor  type  having  a  stationary  armature  and  field  winding,  the 
flux  being  varied  by  a  rotor  of  soft  iron.  (See  photo,  Fig.  67.)  The  open  circuit  voltage 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 

D.C.  LINE 


235 


D.C.  FIELD 
RHEOSTAT 


AUTOMATIC   STARTER, 
l.C.Co.    D.C.  i  K.W.  110 
VOLT,  TYPE  T"  WITH 
DYNAMIC  BRAKE. 


Fig.   255  —  Fundamental   Wiring   Diagram  of  the  Automatic  Starter  Furnished  With  the  Marconi 

500   Cycle  Transmitter. 


K.   W. 


of  the  generator  is  approximately  350  volts  and  the  load  voltage  about  120  volts.  The 
normal  speed  of  the  machine  is  2,500  R.  P.  M.  The  voltage  at  the  armature  terminals  can 
be  varied  by  the  field  rheostat  mounted  on  the  right  hand  side  of  the  panel.  The  low  power 
circuits  are  protected  by  the  usual  set  of  protective  condensers. 

A  closed  core  transformer  having  a  secondary  potential  of  14,500  volts  is  supplied  and  is 
fitted  with  an  appropriate  safety  gap. 


236 


PRACTICAL  WIRELESS   TELEGRAPHY. 


QUENCHED 
GAP 

GROUND   CONNECTION 

HELD  IN  PLACE 

UNDER  FLANGE  NUT  _^ 

Fig.    256— Radio    Frequency    Circuits    of    the    ^    K.    W.    500    Cycle    Transmitter. 

The  circuits  of  radio-frequency  (Fig.  256)  are  similar  to  the  2  K.  W.  set  with  the  excep- 
tion that  a  condenser  of  fixed  capacity  is  employed  throughout  all  wave  lengths.  Also  the 
secondary  winding  is  moved  away  from  the  primary  for  change  of  coupling,  which  is  the 
reverse  of  the  method  employed  in  the  2  K.  W.  set.  However,  the  change  of  wave-length  is 
accomplished  in  the  same  manner  by  a  special  switch  and  the  primary  and  secondary  wind- 
ings remain  in  a  fixed  mechanical  position  for  all  wave-lengths. 

The  transmitter  condenser  consists  of  4  copper-plated  small  Leyden  jars  connected  in 
parallel  having  capacity  of  .001  mfds.  each.  These  are  mounted  in  a  metal  rack  behind  the 
panel. 

This  set  is  also  fitted  with  a  single  pole  double  throw  high  potential  switch  which  either 
connects  the  aerial  to  the  transmitter  or  connects  it  directly  to  earth  for  protection  against 
lightning.  This  switch  operates  independently  of  the  antenna  changeover  switch. 

The  automatic  starter  supplied  with  this  set  has  been  shown  diagrammatically  in  Fig.  74, 
but  the  diagram,  Fig.  255,  outlines  the  circuits  more  as  they  appear  on  the  actual  panel. 
The  starter  consists  essentially  of  two  magnets  fitted  with  armatures,  one  of  which  con- 
nects the  motor  armature  to  the  D.  C.  line  through  a  single  resistance  coil  and  the  other  cuts 
out  this  resistance  thereby  connecting  the  motor  direct  to  the  D.  C.  line.  Appropriate  resist- 
ance coils  are  connected  in  series  with  the  solenoid  windings  automatically,  when  the  starter 


MOTOR 
RHEOSTAT 


AERIAL  TUNING 
INDUCTANCE " 


WAVE  LENGTH 
SWITCH  — f 


AUTOMATIC' 


-20| 

WATTMETER 


D  C 


A.C 


GENERATOR 
RHEOSTAT 


AERIAL 
AMMETER 


LIGHTNING 
"    SWITCH 


COUPLING 
ADJUSTER 


QUENCHED 
GAP 


ROTARY 
GAP 


Fig.   257— Front  View  of  the   Marconi    y2   K.   W.   500  Cycle  Transmitter. 


238 


PRACTICAL   WIRELESS   TELEGRAPHY. 
16" 


TUNING 


WAVE  LENGTH     x 
CHANGING  SWITCH 


PRIMARY        SECONDARY 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


239 


CONDENSIR 


is  in  the  full  running  position.  This 
prevents  the  magnet  windings  over- 
heating as  the  potential  difference 
across  the  motor  armature  rises.  A 
dynamic  brake  attached  to  the  motor 
starter  consists  of  a  shunt  armature 
resistance,  which  is  connected  across 
its  terminals  by  a  special  set  of  con- 
tacts attached  to  the  starter. 

A  2-pole  single-throw  switch  dis- 
connects the  motor  from  the  power 
line.  A  single-pole  switch  permits  the 
circuit  of  the  generator  field  to  be 
interrupted  as  a  matter  of  safety  dur- 
ing adjustment  of  the  set  and  a  sec- 
ond D.  P.  S.  T.  switch  breaks  the 
circuit  from  the  generator  to  the 
transformer. 

The  remainder  of  the  equipment 
consists  of  a  Type  I  aerial  switch. 
Type  106  receiving  tuner  and  flush 
sivitch  for  starting  and  stopping  the 
motor  generator  from  a  distant  point. 
Also  an  aerial  ammeter,  range  0-10 
amperes,  and  a  direct  reading  watt- 
meter, 0-750  watts. 

191.  Complete  Adjustment 
of  the  Type  P-5  Set— (a)  Tun- 
ing. The  tuning  of  this  set,  the  ad- 
justment of  the  frequency  and  the 
voltage  of  the  alternator  and  the  ad- 
justment of  the  rotary  gap  is  prac- 
tically the  same  as  the  2  k.  w.  500 
cycle  set  described  in  previous  para- 
graphs. The  adjustment  of  the 
quenched  gap  is  identical  with  the 
method  described  for  the  2  k.  w.  set 
in  section  b,  paragraph  188. 

(6)  Motor  Starter.  The  opera- 
tion of  the  y*  k.  w.  starter  shown  in 
Fig.  260  is  as  follows :  The  cut-out 
switches  of  the  automatic  starter  are 
energized  by  direct  current  from  the  main  D.  C.  line  and  operated  by  a  distant  control  switch. 
When  this  switch  is  closed,  the  solenoid  "A"  is  energized  and  moves  the  contact  arm  C 
radially  in  a  vertical  plane,  perpendicular  to  the  board,  which  cuts  in  the  single  resistance 

unit  mounted  in  the  re- 
sistance box  behind  the 
panel. 

As  the  speed  of  the 
motor  generator  in- 
creases, the  current 
falls  off,  and  the  poten- 
tial difference  across 
the  armature  rises. 
When  it  reaches  normal 
value,  the  solenoid  B  is 
energized  and  draws 
the  contact  arm  F,  cut- 
ting out  the  resistance. 
This  connects  full  line 
voltage  across  the 
motor. 


Fig.    259— Rear    Vie\ 


j    of    the    Marconi    ' 
Panel    Transmitter. 


K.   W.    500    Cycle 


D.C.UNE 


AC  GEN 


P<    Ps   P&   PT 
©000 


Fig.    260—  Automatic    Starter    Panel 


K.    W.    500    Cycle    Transmitter). 


240 


PRACTICAL   WIRELESS   TELEGRAPHY. 


CUT  OUT  FOR    SHORT 
WAVE    CONDEN5ER 


LOADING  COIL 


MOTOR  FIELD  GENERATOR 

RHEOSTAT  FIELD 

RHEOSTAT 


ABC     ARE     N°  5    MOUNTED 
INSULATORS.  D    15    PLUG 
THIS   IS    TO    CHANGE    OVER 
FROM  COIL  TO  POWER    AND 
VICE    VERSA 


IZ  CELLS 
STORAGE  BATTERY 


Fig.    261 — Complete    Wiring    Diagram    of    Marconi    1    K.    W.    60    Cycle    Non-synchronous    Transmitter. 

The  time  of  closing  the  contacts  of  the  automatic  starter  is  regulated  by  the  set  screw  E, 
Fig.  260,  attached  to  the  bottom  of  the  starter  arm  B.  Both  solenoids  should  perform  their 
functions  within  six  seconds  from  the  time  the  starting  switch  is  closed.  Like  in  the  2  k.  w. 
set,  the  motor  starter  is  controlled  by  a -special  set  of  contacts  mounted  on  the  aerial  change- 
over switch  or  the  motor  may  be  held  in  a  continuous  state  of  operation  by  closing  the  flush 
switch  mounted  near  to  the  antenna  switch.  In  the  later  models  of  this  set,  when  the 
starter  is  in  full  running  position,  resistances  are  cut  in  automatically  at  G  and  H  to  prevent 
the  solenoid  windings  overheating. 

192.  How  to  Remove  the  Armature  of  the  y2  K.  W.  Motor  Generator. — 

To  remove  the  armature  for  purpose  of  repair  or  replacement,  it  is  necessary  first  to  remove 
the  casing  of  the  spark  gap,  then  remove  the  disc,  after  which  the  bearing  bracket  can  be 
removed  from  the  generator.  Next  remove  the  motor  brushes  after  which  the  armature 
can  be  pulled  out.  When  replacing  the  armature  the  oil  rings  should  be  held  up  to  permit 
the  shaft  to  pass  through  the  bearings.  The  oil  rings  should  then  be  placed  in  their  slots 
making  sure  that  they  move  freely. 

To  remove  the  disc  of  the  rotary  gap,  the  small  machine  screw  in  the  shaft  key  should  be 
unscrewed  until  it  turns  freely.  A  slight  tap  at  the  hub  will  then  loosen  the  key  which  may 
be  removed  by  a  pair  of  pliers.  After  this  a  slight  tap  on  the  hub  of  the  disc  will  loosen  the 
other  half  of  the  key  which  may  then  be  removed.  The  disc  should  then  slip  off  readily. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


241 


193.  The    1  K.    W.    Non-Synchronous    Discharger    Transmitter. Certain 

ships  in  the  Marconi  service  are  fitted  with  a  type  of  transmitter   which  originally  was 
designed  to  be  operated  from  a  60  cycle  motor  generator,  with  a  plain  spark  gap,  but  the  set 
is  now  fitted  with  a  non-synchronous  rotary  discharger  by  which  the  effectiveness  and  range 
has  been  materially  increased.     A  complete  circuit  diagram  of  this  set  is  shown  in  Fig.  261 
and  the  lay-out  of  the  transmitter  and  receiver  in  a  silence  cabin  in  Fig.  262. 

194.  Description   of  the   Set. — The   motor   generator   for   this   set   is   the 
"Standard"  Robbins  and  Meyers  type.    The  motor  is  shunt  wound  for  110  volt  direct  current. 

The  generator  is  of  1  K.  W.  capacity  and  has  an  accumulative  compound  field  winding 
which  tends  to  maintain  a  constant  A.  C.  voltage  under  variable  load.  The  generator  delivers 
current  at  110  volts,  60  cycles.  (See  Fig.  65;  also  Fig.  79a.) 

The  circuit  to  the  transformer  is  interrupted  by  a  type  C  standard  transmitting  key. 

The  power  transformer  is  of  the  open  core  type  and  is  designed  to  operate  on  110  volt  60 
cycle  circuits  without  external  reactance.  The  secondary  voltage  is  approximately  18,000 
volts. 


^STARTER 

-MAIN  LINE  SWITCH 


Fig.    262     The    Installation    of    a    1    K.    W.    Transmitter   in    a    Silence   Cabin. 

A  hand-operated  motor  starter  and  two  rotary  field  rheostats  are  supplied.  The  latter  are 
the  Cutler-Hammer  type. 

The  high  potential  condenser  comprises  6  copper-plated  jars  of  .002  mfds.  each,  con- 
nected in  parallel,  mounted  in  a  metal  rack  and  insulated  from  the  operating  table  by  corru- 
gated porcelain  insulators.  Connection  between  the  outside  coatings  is  established  through  the 
frame  of  the  rack.  Only  four  jars  are  required,  the  remaining  two  being  used  as  spares. 

The  oscillation  transformer  has  been  illustrated  in  Fig.  119.  The  primary  winding  is 
made  of  copper  tubing  mounted  on  porcelain  insulating  supports.  Tappings  may  be  taken 
from  the  primary  inductance  by  means  of  special  spring  contact  clips.  The  secondary  wind- 
ing has  a  fixed  number  of  turns  wound  about  an  insulating  drum  which  slides  vertically  on 
a  brass  tube.  One  terminal  of  this  winding  is  connected  to  the  tube  and  the  opposite  terminal 
to  a  terminal  of  the  aerial  tuning  inductance. 

The  aerial  tuning  inductance  consists  of  a  number  of  turns  of  copper  cable  wound  on  and 
insulated  from  a  wooden  frame.  The  required  number  of  turns  for  any  particular  wave- 
length are  cut  in  by  means  of  a  contact  clip  attached  to  a  flexible  lead. 

The  short  wave  condenser  consists  of  four  copper-plated  jars  mounted  on  a  rack  insulated 


242 


PRACTICAL   WIRELESS   TELEGRAPHY. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO.  243 

by  corrugated  porcelain  insulators.     Each  jar  has  capacity  of  .002  mfds.,  consequently  the 
four  in  series  have  capacity  of  .0005  mfds. 

The  remainder  of  the  equipment  consists  of  a  Type  106  Tuner,  head  telephone,  Type  I 
aerial  change-over  switch,  and  the  necessary  aerial  equipment. 

195.  Installation. — The  component  parts  of  the  closed  oscillation  circuit 
being  mounted  separately,  short  connecting  leads  must  be  employed  to  connect  up  the  con- 
denser, oscillation  transformer  and  spark  gap.     Otherwise,  it  will  be  difficult  to  secure  the 
wave-length  of  300  meters.    Perhaps  the  best  arrangement  of  the  apparatus  is  that  shown  in 
Fig.  262,  where  the  transmitting  apparatus  is  installed  in  a  silence  cabin,  the  operating  room 
having  the  receiving  tuner,  the  motor  starter,  rheostats,  the  transmitting  key  and  the  aerial 
change-over  switch. 

In  the  transmitting  cabin,  the  rotary  gap  is  mounted  atop  the  high  potential  transformer ; 
the  oscillation  transformer  and  condenser  on  the  table  immediately  above. 

The  motor  starter  should  be  installed  where  it  is  convenient  to  the  left  hand  of  the 
operator. 

Lead-covered  cable  is  used  to  connect  up  the  power  circuits,  No.  10  S.  B.  R.  C.  being 
used  for  the  circuits  carrying  heavy  current,  and  No.  14  S.  B.  R.  C.  for  the  field  circuits  and 
rotary-gap  motor.  Copper  tubing  is  generally  used  for  the  radio-frequent  circuits  and  copper 
ribbon  for  the  earth  wire.  The  receiving  tuner  is  connected  to  the  aerial  switch  by  No.  18 
fixture  wire. 

196.  Adjustment  of  the  1  K.  W.  Set. — (a)  Regulation  of  Voltage  and  Fre- 
quency.   Care  should  be  exercised  not  to  overload  this  set  by  the  use  of  abnormal  voltages 
at  the  generator.    These  machines  are  constructed  so  that  when  110  volts  D.  C.  is  supplied 
to  the  motor  110  volts  A.  C.  is  supplied  by  the  generator.    The  contact  on  the  generator  field 
rheostat  should  occupy  an  approximate  midway  position  for  the  correct  voltage  while  the 
motor  rheostat  should  be  set  at  the  zero  position;  e.  g.,  all  resistance  cut  out  of  the  circuit 
(when  the  voltage  of  the  ship's  generator  is  normal). 

(&)  Rotary  Gap.  The  arm  of  the  rotary  gap  should  be  driven  at  approximately  2,400 
R.  P.  M.,  but  should  be  regulated  according  to  the  pitch  of  the  note;  in  other  words,  the 
speed  giving  the  clearest  note  is  the  one  to  use.  A  small  speed  controlling  rheostat  is 
mounted  in  the  instrument  room  convenient  to  the  operator.  The  adjuster  should  work 
in  the  direction  of  obtaining  a  note  of  high  pitch  with  low  values  of  voltage  at  the  generator 
rather  than  with  excessively  high  voltages.  The  gap  between  the  points  of  the  rotor  and 
those  on  the  stationary  arm  should  be  adjusted  to  a  minimum  length. 

(c}  Tuning.  The  tuning  of  this  set  is  accomplished  by  first  tuning  the  closed  circuit  to 
one  of  the  standard  waves  with  a  wavemeter,  followed  by  a  similar  adjustment  of  the  aerial 
circuit.  The  better  method  is  to  excite  the  antenna  with  a  spark  coil  or  vibrating  buzzer 
and  in  this  manner  measure  the  natural  wave  length  of  the  aerial,  then  add  turns  at  the 
aerial  tuning  inductance  until  the  wave-length  of  600  meters  is  obtained.  Proceed  similarly 
for  300  meters,  but  the  short  wave  condenser  must  be  connected  in  series  with  the  aerial 
and  perhaps  another  selection  of  turns  made  at  the  aerial  tuning  inductance. 

The  degree  of  coupling  and  the  consequent  purity  of  the  radiated  wave  may  be  adjusted 
by  raising  or  lowering  the  secondary  winding  in  respect  to  the  primary  winding,  on  the 
vertical  rod  or  post.  The  secondary  winding  may  be  clamped  permanently  at  the  correct 
position  of  coupling. 

(d)  The  type  106  receiving  tuner  circuits  and  adjustment  are  completely  covered  in  para- 
graph 138,  Part  IX. 

(c )  Tuned  Coil  Emergency  Transmitter.  The  student  should  note  that  the  diagram,  Fig. 
261,  contains  the  complete  circuits  of  the  Marconi  tuned  coil  emergency  transmitter.  The 
diagram  includes  the  wiring  of  the  induction  coil  and  the  charging  panel. 

When  the  left  hand,  two  blade  switch  on  the  charging  panel  is  closed,  the  main  D.  C. 
line  to  the  batteries  is  closed,  but  current  will  not  flow  through  the  batteries  until  the 
plunger  of  the  underload  circuit  breaker  is  pushed  up  by  hand.  If  there  are  no  loose  con- 
nections in  the  charging  circuit,  the  plunger  will  remain  in  this  position  until  the  charging 
current  is  cut  off  or  until  the  voltage  of  the  generator  drops  below  that  of  the  batteries 
(note  position  of  these  switches  in  Fig.  205a,  Chapter  10). 

In  order  to  take  readings  of  the  battery  voltage,  a  small  strap  key  placed  alongside  the 
underload  circuit  breaker  must  be  closed. 


244 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


When  the  right  hand  double  pole  double  throw  switch  is  thrown  to  the  upper  set  of  con- 
tacts, the  transmitting  key  is  connected  in  series  with  the  power  transmitting  set,  but  if 
thrown  in  the  opposite  position  the  key  is  in  series  with  the  primary  circuit  of  the  induction 
coil. 

To  place  the  coil  transmitter  into  operation,  the  flexible  plug  contact  attached  to  B 
should  be  removed  from  A  and  connected  into  C. 

The  arm  of  the  rotary  gap  should  be  stopped  in  such  position  that  a  gap  of  the  correct 
length  is  provided  for  the  induction  coil. 

The  student  should  also  trace  out  (in  this  diagram)  the  circuit  of  the  rotary-gap  motor 
which  includes  the  two-blade  starting  switch  and  the  series  rheostat.  In  practice  the  rheostat 
is  adjusted  until  a  clear  musical  tone  is  obtained  at  the  gap. 

The  motor  starter  shown  in  this  diagram  is  the  standard  General  Electric  Company's 
hand  starter,  the  circuits  of  which  are  fully  shown  in  Fig.  72. 

197.  Type  "E-2"   One-Half  Kilowatt,   120  Cycle   Panel  Transmitter.— A 

number  of  vessels  in  the  American  Mar- 
coni service  are  equipped  with  *^>  k.  w., 
120  cycle  panel  transmitters  fitted  with 
a  quenched  gap  but  unlike  the  type 
"P-4"  and  type  "P-5,"  this  set  is  de- 
signed for  the  two  standard  wave- 
lengths only,  namely,  300  and  600 
meters.  For  emergency  use,  a  rotary 
spark  gap  is  mounted  on  the  end  of  the 
motor  generator  shaft,  which  may  be 
used  in  place  of  the  quenched  gap  when- 
ever required.  A  complete  circuit  dia- 
gram of  the  type  "P"  set  appears  in 
Fig.  263,  a  front  view  in  Fig.  264  and 
rear  views  in  Figs.  265  and  266. 

A  number  of  spare  parts,  such  as  an  extra 
motor  generator  armature,  motor  blower 
armature,  quenched  gap  plates,  condenser 
jars,  etc.,  are  supplied,  positively  insuring 
against  all  possibility  of  breakdown. 

198,  Details  of  the  Circuits  and 
Apparatus. — Explanation  of  the  de- 
tailed wiring  diagram  shown  in  Fig.  263  fol- 
lows : 

The.  motor  generator  is  of  the  Eck  type, 
having  a  two  pole  shunt  wound  motor  and  a 
simple  shunt  field  winding  for  the  generator. 
The  speed  of  the  motor  is  regulated  by  the 
sliding  contact  rheostat  R  and  the  voltage  of 
the  generator  altered  by  the  rheostat  R-l.  The  generator  field  circuit  may  be  broken  by  the 
small  switch  shown  in  the  drawing.  The  receiving  apparatus  is  thus  protected  from  'hum- 
ming" noises  due  to  induction  from  the  A.  C.  generator  circuit. 

The  motor  starter  is  the  Cutler-Hammer  single  step  type  and  is  constructed  somewhat 
differently  than  the  types  employed  in  the  2  K.  W.  and  %  K.  W.  500  cycle  panel  transmitters. 
When  the  main  D.  C.  line  switch  is  closed,  the  resistance  coil  S  is  connected  in  series  with 
the  D.  C.  armature.  Owing  to  the  almost  complete  absence  of  counter  electromotive  force, 
the  potential  difference  across  the  terminals  of  the  armature  at  the  start  of  the  motor  is 
low;  hence  insufficient  current  flows  through  the  solenoid  windings  W  to  draw  up  the 
plunger  P.  As  the  speed  of  the  armature  increases,  the  counter  electromotive  force  rises, 
and  accordingly  the  difference  of  potential  increases ;  finally  sufficient  current  flows  through 
the  solenoid  W  to  draw  up  the  plunger  P,  whereupon  the  contacts  C  and  C-l  are  short- 
circuited,  shunting  the  resistance  coil  S  out  of  the  circuit.  The  motor  is  now  connected 
directly  to  the  D.  C.  line. 

The  plunger  P  also  separates  the  contacts  D  and  D-\  connecting  the  resistance  coil  5-1 
in  series  with  the  winding  W  to  protect  the  latter  from  overheating  or  burning  out. 


Fig.    264 — Front   View   of   the   Marconi    l/2   K.    W. 
120   Cycle   Transmitter. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


245 


AERIAL  TUNING 
INDUCTANCE 


The  transformer  of  the  closed  core  type  is  denoted  by  the  primary  winding  T  and  the 
secondary  winding  T-l.  Just  previous  to  pressing  the  key,  the  voltage  of  the  generator  is 
about  300  volts,  but  immediately  drops  to  approximately  110  volts  when  the  primary  circuit 
of  the  transformer  is  closed.  The  secondary  voltage  of  the  transformer  with  the  con- 
densers in  shunt  is  about  14,700  volts.  A  safety  discharge  gap  is  connected  to  the  secondary 
winding  terminals  for  the  protection  of  the  condensers  and  transformer  in  case  one  of  the 
leads  from  the  regular  discharge  gap  should  become  disconnected. 

The  wattmeter  of  the  ordinary  switchboard  type— range  0-750  watts— is  connected  in  the 
circuit  from  the  alternator  to  the  transformer  primary.  The  resistance  coil  N  placed  in 
series  with  the  potential  coil  of  the  meter  is  mounted  at  the  front  base  of  the  switchboard. 

The  condensers  are  of  the  tubular  Leyden  jar  type  plated  inside  and  outside  with  copper. 
The  average  capacity  of  each  jar  is  .0015  mfd.    The  actual  value  is  accurately  measured  and 
marked   directly  on  the  glass   of   each  jar.     In   case   of   puncture  a  condenser  jar  of   like 
capacity  must  be  substituted.    The  seven  Ley- 
den    jars    connected    in    parallel   give   a   total 
capacity  of  about  .011   mfd.     The  inside  and 
outside  terminals  of  the  condensers  are  con- 
nected directly  to   the   secondary   winding  of 
the  transformer. 

The  oscillation  transformer  consists  of  a 
primary  and  secondary  winding  of  the  pancake 
type,  which  are  indicated  at  L  and  L-l  re- 
spectively. Each  winding  consists  of  a  single 
coil  of  copper  ribbon  properly  mounted  and 
spaced  on  an  insulated  base. 

The  secondary  winding  L-l,  mounted  on  a 
movable  base,  permits  the  coupling  between  it 
and  the  primary  winding  L  to  be  quickly  and 
easily  adjusted. 

The  wave-length  changing  switch,  operated 
by  the  handle  H,  has  two  single  blade  contacts 
which  when  thrown  to  the  right  simultaneous- 
ly connect  such  values  of  inductance  in  the 
antenna  circuit  and  closed  oscillation  circuit  as 
to  give  each  circuit  a  period  corresponding  to 
600  meters.  When  thrown  to  the  left  both 
circuits  are  given  a  wave-length  of  300  meters, 
provided  the  short-circuiting  switch  of  the 
short  wave  condenser  X  is  open. 

The  correct  number  of  primary  and  second- 
ary turns  for  the  300  and  600  meter  waves  are  MOTOR 
determined  by  means  of  a  wavemeter  and  con- 
nection therefor  made  through  the  flexible 
connectors  E  and  E-l.  The  taps  on  the  sec- 
ondary inductance  are  located  in  the  same 
manner.  Two  values  of  inductance  for  the 
standard  wave  lengths  are  selected  through  the  flexible  conductors  F  and  F-l. 

The  spark  discharger  is  of  the  multiple  plate  series  type  (quenched  gap),  consisting  of 
fifteen  plates  giving  fourteen  gaps.  No  more  than  eight  of  these  are  generally  required, 
leaving  a  number  of  spares.  The  discharge  plates  are  of  copper  carefully  ground,  presenting 
an  absolutely  smooth  and  uniform  sparking  surface.  The  plates  are  separated  by  specially 
treated  fibre  washers.  Cooling  is  effected  by  means  of  a  small  direct  current  blower  J.  The 
motor  circuit  includes  a  switch  for  starting  and  stopping  purposes. 

The  synchronous  rotary  gap  discharger  Y  is  mounted  on  the  end  of  the  motor  generator 
shaft.  The  rotating  member  has  six  discharge  electrodes,  while  the  stationary  electrodes  are 
two  in  number.  The  stationary  electrodes  may  be  shifted  through  a  small  arc,  thereby 
permitting  a  spark  discharge  for  each  alternation  of  the  charging  current.  This  produces  a 
discharge  of  uniform  pitch  having  a  musical  characteristic.  The  group  frequency  of  this  set 
is  240  sparks  per  second. 


Fig.    265— Rear    View    of    the    Marconi    ]/2    K.    W. 
120    Cycle    Transmitter. 


246 


PRACTICAL  WIRELESS  TELEGRAPHY. 


c 


The  aerial  tuning  inductance  of  the  continuously  variable  type,  represented  at  L-2,  is  con- 
nected in  series  with  the  earth  lead  and  consists  of  a  single  spiral  of  copper  ribbon  having  a 
sliding  contact  U  which  allows  connection  to  be  made  at  any  point  on  the  spiral. 

The  aerial  changeover  switch  for  shifting  the  antenna  from  the  sending  to  the  receiving 
apparatus  consists  of  a  single  blade  double  throw  switch.  When  thrown  to  the  right,  the 
antenna  is  connected  directly  to  the  terminals  of  the  receiving  tuner ;  when  thrown  to  the 
left,  connection  is  made  direct  to  the  secondary  winding  of  the  transmitting  oscillation  trans- 
former. These  sets  may  be  supplied  with  the  type  S,  S,  H,  or  I  aerial  changeover  switch 
as  well. 

The  aerial  ammeter  of  the  Roller-Smith  type,  having  a  range  of  zero  to  10  amperes,  is 
permanently  connected  in  the  aerial  circuit  and  is  used  for  obtaining  the  maximum  value  of 
current  and  to  indicate  conditions  of  resonance  between  the  open  and  closed  oscillation 
circuits. 

The  short  wave  condenser  is  of  the  standard  Marconi  type  and  may  consist  of  either  four 

flat  plates  of  glass  connected  in  series  or  four 
Leyden  jars  connected  in  series,  giving  a  re- 
sultant capacity  of  .0005  mfd. 

The  low  potential  power  circuits  are  pro- 
tected from  electrostatic  induction  by  pro- 
tective condensers  of  large  capacity.  Two  of 
these  condensers  are  connected  in  series  and 
earthed  at  the  middle  point.  These  protective 
units  are  connected  across  the  generator  field 
windings,  the  motor  field  windings,  the  arma- 
ture of  the  blower  motor  and  the  alternating 
current  armature.  The  frame  of  the  motor 
generator  is  connected  directly  to  earth. 

199.  General  Instructions  for  Tun- 
ing and  Adjusting. — The  following 
general  instructions  should  be  carefully 
observed  by  inspectors  and  operators: 

(a)  Regulation  of  Voltage  and  Frequency. 
Connect  the  D.  C.  line  (110-120  volts)  to  the 
lower  terminal  of  the  fused  switch  marked 
"D.  C.  Line,"  the  aerial  to  the  binding  post 
marked  "A"  on  the  back  of  the  antenna  change- 
over switch  and  ground  either  one  or  both  of 
the  terminals  of  the  iron  frame  which  sup- 
ports the  panel. 

The  slider  of  the  generator  field  rheostat 
should  be  placed  near  the  lower  end,  the  an- 
tenna switch  turned  to  "Send"  position  and 
about  eight  or  nine  gaps  of  the  quenched  gap 
connected  in  the  circuit. 

Next  observe  that  none  of  the  leads  be- 
tween the  double  two-point  wave  length  switch 
and  the  oscillation  transformer  are  discon- 
nected. Close  the  D.  C.  line  switch  which  starts 
the  motor  generator  through  the  automatic  starter.  Then  close  the  "generator  field"  and 
the  "blower  motor"  switch;  finally  close  the  "A.  C.  line"  switch. 

(6)   Tuning.    The  complete  set  is  now  ready  for  tuning.    Disconnect  the  antenna  and  tune 
the  closed  oscillatory  circuit  to  the  desired  wave  length  by  means  of  a  wavemeter. 
Set  the  two-point  switch  indicator  at  "300." 

Adjust  the  tap  leading  from  the  front  coil  of  the  oscillation  transformer  to  switch  point 
marked  "300"  to  the  300  meter  wave  length.  Then  turn  the  indicator  to  "600"  and  adjust  the 
tap  to  the  higher  wave  length. 

Next  connect  the  set  with  the  antenna,  tune  the  secondary  or  open  circuit  to  the  closed 
circuit  and  locate  taps  for  each  of  the  two  standard  wave  lengths. 

The  coupling  as  well  as  the  sliding  contact  which  travels  over  the  turns  of  the  aerial 
tuning  inductance,  should  be  varied  while  locating  the  wave-length  tap,  to  find  the  best  posi- 


Fig.    266— Rear   View   of    the    Marconi 
120    Cycle   Transmitter. 


K.   W. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


247 


tion  of  both  the  coupling  and  flexible  connection.  The  circuits  are  in  resonance  when  the 
aerial  ammeter  shows  the  highest  possible  reading. 

With  the  average  ship's  aerial  it  is  possible  to  locate  the  correct  number  of  turns  at  the 
secondary  winding  of  the  oscillation  transformer  so  that  the  radiated  wave  can  be  changed 
from  600  meters  to  300  meters  by  simply  throwing  the  changeover  switch  to  the  proper  posi- 
tion. When  this  switch  is  in  the  300  meter  position,  the  short  wave  condenser  which  is  placed 
in  series  with  the  earth  lead  must  have  its  short-circuiting  strap  removed.  Generally  speak- 
ing, the  aerial  tuning  inductance  tap  is  placed  at  a  fixed  point  for  either  the  600  meter  or  300 
meter  wave. 

(c)  Adjustment  of  the  Spark  Note.  Either  a  receiver  or  a  wavemeter  may  be  used  to  indi- 
cate the  quality  of  the  note.  If  the  note  is  not  clear  vary  the  generator  voltage,  the  number 
of  gaps  and  the  transformer  coupling,  until  it  is  clear  or  has  a  pitch  of  about  240  sparks  per 


Fig.   266a— 2   K.   W.   240  Cycle  Motor   Generator  With   Synchronous  Disc  Discharger. 

second.  If  the  generator  voltage  is  too  high  or  too  low,  the  note  will  either  be  rough  or 
else  have  the  wrong  pitch.  The  note  may  also  be  judged  by  the  tone  of  the  brush  discharge 
of  the  condensers. 

Maximum  antenna  current  does  not  indicate  maximum  efficiency.  The  note  must  also  be 
kept  clear  and  the  gap  not  worked  above  the  antenna  current  at  which  the  clearness  is  not 
maintained. 

The  wattmeter  should  indicate  about  500  watts  and  the  number  of  gaps  used  should  be 
between  eight  and  twelve. 

The  power  of  the  set  varies  in  the  square  of  the  number  of  gaps ;  that  is,  if  the  ten  gaps 
represent  full  power  the  power  would  be  102  =  100. 

The  set  is  designed  to  carry  a  load  of  500  watts.  Any  overload  is  liable  to  develop  volt- 
ages that  may  break  down  the  condensers  or  cause  trouble  elsewhere  in  the  set. 

(d)  Adjustment  of  the  Quenched  Gap.  The  quenched  gap  should  not  be  taken  apart  until 
absolutely  necessary.  When  the  circuits  are  in  resonance,  but  the  antenna  current  is  less  than 
5  to  7  amperes  or  the  wattmeter  reading  falls  below  its  usual  value,  or  when  unable  to  get  a 
clear  note,  the  gaps  should  be  opened  and  the  spark  surfaces  examined. 

If  the  gap  becomes  short-circuited  it  can  be  determined  by  the  use  of  the  "gap  tester" 
(an  insulated  rod  having  a  brass  piece  inserted  in  one  end),  which  will  indicate  no  'spark 
when  bridged  across  two  adjacent  spark  discs. 

To  open  the  gap,  loosen  the  set  screw  in  the  left  hand  of  the  gap  with  a  wrench  which  is 
supplied  with  the  set.  Then  lift  out  the  plates. 


248  PRACTICAL   WIRELESS   TELEGRAPHY. 

If  the  plates  and  gaps  stick  together  take  care  when  forcing  them  apart  not  to  injure  the 
gasket  or  the  sparking  surfaces.  Should  the  gasket  become  injured  or  cling  to  the  metal, 
clean  the  plate  off  carefully  and  insert  a  new  gasket. 

The  sparking  surface  of  the  plate  should  have  a  light,  pink  color  with  a  somewhat  dull 
finish.  If  a  plate  has  a  rough,  black  appearance,  it  indicates  that  the  gap  was  not  airtight  and 
the  sparking  surfaces  should  be  carefully  cleaned  with  a  very  fine  emery  cloth. 

Do  not  operate  the  set  unless  the  blower  motor  of  the  gap  is  running,  otherwise  the  heat 
may  destroy  the  gaskets.  Never  touch  any  circuit  which  may  be  alive  without  first  opening 
the  field  switch  or  the  generator  main  switch,  or  possibly  both. 

200.  Marconi  2  K.  W.  240  Cycle  Transmitter. — A  number  of  vessels  in  the 
Marconi  service  are  fitted  with  a  Z  k.  w.  synchronous  spark  transmitter  consisting 
of  several  isolated  instruments  mounted  on  an  operating  table  in  a  convenient 
way. 

The  motor  generator  is  a  special  Eck  machine  which  takes  current  at  the  motor  at  110 
volts  and  generates  current  at  500  volts  at  frequency  of  240  cycles.  The  motor  is  of  the 
interpole  type  which  gives  sparkless  commutation  and  constant  speed  regulation  under 
variable  load. 

A  synchronous  rotary  spark  discharger,  mounted  on  the  generator  shaft,  consists  of  a 
fiber  disc  carrying  twelve  spark  points  connected  together  by  a  copper  strip.  The  stationary 
electrodes,  two  in  number,  are  mounted  on  a  wooden  muffling  drum  which  encloses  the  disc. 
Synchronous  discharges  are  obtained  by  shifting  the  drum»  carrying  the  stationary  electrodes, 
one  way  or  the  other,  until  a  uniform  spark  note  is  secured.  Simultaneous  adjustments  of 
the  generator  voltage  must  be  made  noting  the  reading  of  the  ammeter  to  insure  that  normal 
power  consumption  is  not  exceeded.  The  complete  motor  generator  and  spark  gap  is 
shown  in  Fig.  266a.  The  condenser  may  be  either  of  the  flat  plate  glass  or  copper-plated  jar 
.  type.  The  oscillation  transformer  is  the  type  mentioned  in  connection  with  the  1  k.  w.  60 
cycle  sets  and  shown  in  the  photograph,  Fig.  119. 

An  open  core  transformer  is  employed  taking  current  in  the  primary  at  500  volts  and 
delivering  at  the  secondary,  current  at  pressure  of  15,000  volts. 

The  receiving  equipment  may  consist  either  of  type  107a  or  a  type  106  tuner  connected  to 
a  type  I  or  type  S.  H.  aerial  changeover  switch. 

Either  a  General  Electric  Company  or  Cutler-Hammer  Hand  Starter  may  be  supplied 
with  the  motor  generator. 

Since  the  process  of  tuning  of  radio  sets  has  been  gone  over  repeatedly  in  connection 
with  other  sets  of  the  Marconi  Company  and  particularly  in  paragraph  164,  it  is  not  deemed 
necessary  to  repeat  the  instructions  again.  Briefly,  the  rotary  gap  is  adjusted  for  a  cleat- 
spark  note  after  which  the  closed  circuit  is  tuned  to  the  standard  wave  by  a  wavemeter  and 
the  antenna  circuit  is  adjusted  to  resonance  by  a  hot  wire  ammeter  or  by  the  wavemeter  if 
desired. 

201.  Type  P-9,  ^  K.  W.  Cargo  Transmitting  Set.— One  of  the  latest  types 
of  transmitters  developed  by  the  Marconi  Wireless  Telegraph  Company  of  America 
is  the  type  P-9  set  which  was  primarily  intended  for  cargo  vessels  or  ships  of 
small  tonnage. 

Although  the  apparatus  of  this  set  is  not  placed  on  a  panel  board  as  the  types 
P-4  and  P-5,  the  component  parts  are  mounted  in  a  convenient  way  in  an  angle 
iron  frame  as  shown  in  the  photographs,  Figs.  266-b  and  266-c.  Fig.  266-d  is  a 
complete  wiring  diagram  of  the  set. 

It  is  to  be  noted  first  that  the  lower  part  of  the  frame  holds  the  motor  generator,  the  high 
voltage  condenser  and  a  step-up  closed  core  transformer. 

On  the  upper  shelf  is  placed  the  oscillation  transformer,  the  aerial  tuning  inductance,  the 
short  wave  condenser,  the  lightning  switch  and  a  resonance  indicator  for  purposes  of  tuning. 

A  hand-operated  motor  starter  is  mounted  to  the  side  of  the  panel  and  immediately  under- 
neath a  terminal  board  for  making  connections  to  the  power  mains,  the  aerial  changeover 
switch  and  the  telegraph  key. 

This  set  was  designed  with  the  utmost  simplicity  of  construction  and  operation  in  view 
and  is  one  that  can  be  placed  in  unskilled  hands  without  fear  of  breakdown. 

Unlike  the  Marconi  transmitters  previously  described,  type  P-9  is  not  fitted  with  a 
variable  field  resistance  or  an  aerial  ammeter.  Fixed  resistance  coils  are  connected  in  series 
with  both  the  motor  and  generator  field  windings  and  conditions  of  resonance  between  the 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


249 


open  and  closed  oscillation  circuits  are  determined  by  a  small  glow  lamp  shunted  by  a  semi- 
loop  of  wire.  It  is  to  be  observed  also  that  the  coupling  between  the  primary  and  secondary 
windings  is  changed  by  moving  the  primary  at  right  angles  to  the  secondary. 

(a)  Description  of  the  Set.    The  motor  generator  is  a  ^  K.  W.  500  cycle  machine  of  the 
Crocker-Wheeler  type,  the  load  voltage  of  the  alternator  being  about  120  volts. 


OSCILLAT10H 


HOTARY  GAP 


CLOSED  CORE 
TRANSFORMER 


Fig.   266b— The  Marconi    y4    K.    W.    Cargo   Type  Transmitting  Set   Complete. 

The  motor  has  a  differential  field  winding  for  maintaining  a  constant  speed  under  variable 
load  and  it  is  designed  for  connection  to  a  110  volt  direct  current  circuit. 

The  motor  starter  is  one  of  the  Industrial  Controller  Company's  hand-operated  type  with 
the  usual  connections  to  the  motor.  (See  wiring  diagram,  Fig.  266-d.) 

Protective  V?  microfarad  condensers  are  attached  to  the  motor  generator  circuits  to 
neutralize  any  differences  of  potential  that  may  be  set  up  by  the  transmitter.  One  terminal 
of  these  condensers  is  connected  to  a  binding  post  of  the  machine  and  the  other  terminal  to 
the  frame  of  the  motor  generator,  which,  in  turn,  is  thoroughly  connected  to  earth. 

The  high  voltage  transformer  is  one  of  the  closed  core  type   (Fig.  266c),  which  is  im- 


250 


PRACTICAL  WIRELESS  TELEGRAPHY. 


mersed  in  a  semi-liquid  grease.  A  small  fixed  reactance  coil  is  connected  in  series  with  the 
primary  winding  for  regulation  of  the  current. 

The  high  voltage  condenser  consists  of  two  small  sized  copper-plated  jars  of  .001  micro- 
farads each,  which  are  shunted  across  the  secondary  terminals  of  the  transformer. 

The  oscillation  transformer  has  two  flat  spiral  coils  made  of  copper  strip  wound  edgewise, 
the  primary  winding  being  connected  in  series  with  the  spark  gap  circuit  and  the  secondary 
winding  in  series  with  the  antenna  circuit.  The  correct  number  of  turns  in  either  winding 


AERIAL  TUNING 
INDUCTANCE 


SECONDARY   OSCILLAllOli 
TRANSFORMER 


PRIMARY 

•L, 


TERMINAL 
BOARD 


Fig.    266c — The    Marconi    J4    K.    W.    Cargo    Set    showing    the    General    Arrangement    of 

the    Apparatus. 

for  the  standard  waves  is  obtained  by  means  of  a  small  clip  contact  attached  to  a  flexible 
lead.  The  positions  of  the  300,  450  and  600  meter  waves  at  the  primary  winding  are  clearly 
marked  and  the  contact  clip  must  be  shifted  by  hand  for  each  change  of  wavelength. 

The  aerial  tuning  inductance  also  consists  of  a  flat  spiral  copper  strip  wound  edgewise 
on  an  insulating  base,  the  inductance  being  varied  by  a  flexible  lead  and  contact  clip. 

Instead  of  an  aerial  ammeter,  a  resonance  indicator  is  connected  in  series  with  the 
antenna  circuit.  It  consists  of  a  small  glow  lamp  shunted  by  a  loop  of  wire  upon  which 
bears  a  sliding  contact.  By  regulating  the  length  of  the  shunt  circuit,  the  proportion  of 
current  flowing  through  the  lamp  can  be  carefully  regulated. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO. 


251 


The  short  wave  condenser  consists  of  two  .001  microfarad  condensers  connected  in  series 
which  are  shunted  out  of  the  circuit  by  a  knife  blade  switch.  This  switch  also  connects  the 
aerial  to  earth  for  protection  against  lightning. 

The  type  I  aerial  changeover  switch  furnished  with  this  set  is  fully  described  in  Para- 
graph 155.  This  switch  in  the  transmitting  position: 

(1)  Closes  the  alternating  current  circuit  to  the  transformer; 

(2)  Closes  the  field  circuit  to  the  generator; 

(3)  Disconnects  the  aerial  from  the  receiving  tuner  primary; 

(4)  Places  the  detector,  head  telephone  and  secondary  of  the  receiving  tuner 
on  short  circuit. 


Fig.  266d— Complete  Wiring  Diagram  of  the   *A  K.  W.  Cargo  Transmitting  Set  with  Type  112  Receiving 
Tuner    and    Type    I    Aerial    Changeover    Switch. 

In  the  receiving  position  the  reverse  operations  are  performed. 

The  remainder  of  the  type  P-9  equipment  comprises  a  type  C  transmitting  key,  type  112 
receiving  tuner  and  a  pair  of  2,000  ohms  D.  H.  receivers. 

In  addition,  lead-covered  wire,  rubber-covered  wire  and  copper  tubing  is  supplied  for 
connecting  up  the  apparatus. 

The  type  112  receiving  tuner  is  described  in  detail  in  Paragraph  156. 

(fc)  Installation.  The  main  D.  C.  line  to  the  motor  generator  and  hand  starter  is  inter- 
rupted by  a  30  ampere  knife  blade  switch  which  is  fused  for  20  amperes.  This  is  placed  in  a 
position  convenient  to  the  operator. 

The  blades  of  the  switch  are  connected  to  studs  No.  1  and  No.  2  of  the  terminal  board 
with  No.  14  lead-covered  wire.  Stud  No.  3  of  the  terminal  board  connects  to  one  terminal 


252  PRACTICAL   WIRELESS   TELEGRAPHY. 

of  the  type  C  key;  the  other  key  terminal  connects  to  terminal  No.  12  of  the  type  T  switch. 

Terminal  No.  11  (type  I  switch)  connects  to  stud  6  of  the  terminal  board. . 

Stud  No.  4  of  the  terminal  board  connects  to  terminal  No.  10  of  the  type  I  switch. 

Terminal  No.  7  of  the  type  I  switch  connects  to  stud  No.  5  of  the  terminal  board. 

In  order  to  conduct  induced  currents  to  earth,  the  lead  coating  of  the  wires  is  thoroughly 
connected  to  the  earth  wire. 

The  antenna  lead-in  insulator  is  connected  to  the  type  I  switch  contact  marked  "ANT 
C'K'T"  by  means  of  quarter-inch  copper  tubing. 

The  type'  I  switch  contact  marked  "OSC  C'K'T"  is  connected  to  the  switch  blade  terminal 
of  the  short  wave  condenser. 

All  other  connections  to  the  type  I  switch  are  firmly  soldered  in  special  lugs  provided  for 
the  purpose.  The  copper  tubing  is  supported  by  two  standard  lighthouse  type  of  insulators 
supplied  by  the  Marconi  Company. 

The  ground  stud  on  the  frame  work  of  the  transmitting  apparatus  is  connected  to  earth 
by  a  zinc  strip  one-half  inch  in  width. 

Terminal  No.  1  of  type  I  switch  is  connected  to  binding  post  marked  "Aerial"  or  "ANT" 
on  the  receiving  tuner. 

The  tuner  binding  post  marked  "Ground"  or  "GND"  is  connected  directly  to  the  earth. 

The  left  telephone  binding  post  is  connected  to  the  terminal  No.  5  of  the  type  I  switch. 

The  right  telephone  binding  post  is  connected  to  terminal  No.  2. 

On  the  type  112  receivers  with  serial  numbers  from  2  to  13  inclusive,  the.  small  binding 
post  marked  "Detector"  (located  beside  the  primary  tap  switch)  is  to  be  connected  to  terminal 
No.  4  of  the  type  I  switch;  but  on  receiving  tuners  with  serial  numbers  from  14  up,  the 
binding  post  marked  "Detector"  or  "DET"  opposite  the  telephone  binding  post  is  to  be  con- 
nected to  terminal  No.  4  of  the  type  I  switch. 

The  three  dry  cells  are  connected  to  the  battery  binding  posts  on  the  receiver  as  shown  in 
Fig.  266d.  The  receiving  apparatus  is  connected  with  No.  18  rubber-covered  solid  wire. 

(c)  Adjustment.   After  the  transmitting  set  is  connected  to  the  power  mains,  aerial  switch, 
etc.,  the  first  adjustment  is  to  be  made  at  the  electrodes  of  the  rotary  gap. 

(1)  Lower  the  stationary  electrodes  by  unscrewing  the  adjusting  nut  at  the 
top  (one  at  a  time)  until  they  touch  the  electrodes  on  the  disc;  then  turn 
the  adjusting  screw  to  the  right  until  the  electrodes  just  clear  those  on 
the   rotary  disc,  when  the  latter  is  turned  over  by  hand.      The  proper 
separation  is  from  .005  to  .01  of  an  inch.    Be  sure  that  the  engaging  pin 
on  the  stationary  electrode  is  properly  seated. 

(2)  Next  close  the  main  D.  C.  line  switch  and  start  the  motor  generator  by 
drawing  the  starting  handle  over  slowly. 

(3)  Close  transmitting  key  and  shift  the  muffling  drum  carrying  the  stationary 
spark  electrodes  until  the  note  is  clear  and  musical. 

(d)  Tuning.    The  set  is  tuned  to  the  three  standard  waves  as  follows: 

(1)  Set  contact  clip  on  the  primary  coil  of  the  oscillation  transformer  at  the 
desired  wave  length  (the  position  of  the  tap  for  each  of  the  three  standard 
waves  is  plainly  marked  on  the  coil). 

(2)  To  insure  accuracy,  check  back  the  wave  length  of  the  closed  oscillation 
circuit  by  a  wavemeter. 

(3)  Set  the  primary  coil  of  the  oscillation  transformer  at  an  angle  of  about  50° 
to  the  secondary. 

(4)  Cut  in  4  or  5  turns  at  the  secondary  winding. 

(5)  Add  turns  at  the  aerial  tuning  inductance  until  the  resonance  indicator 
glows  brightly. 

(6)  If  glow  lamp  becomes  excessively   incandescent,  turn  the  small   shunt 
switch  counter  clockwise. 

(7)  Continue  changing  the  antenna  inductance  until  the  indicator  shows  a 
sharp  maximum. 

(8)  Then  draw  the  primary  winding  further  away  from  the  secondary  and 
slightly  retune  the  antenna  circuit. 

(9)  The  set  generally  will  be  properly  tuned  when  a  slight  change  in  coupling 
or  antenna  inductance  causes  a  decrease  of  the  antenna  current. 

(10)  If  the  coupling  proves  too  close  with  the  primary  winding  at  right  angles 
with  the  secondary,  cut  out  turns  in  the  secondary  and  add  turns  at  the 
aerial  tuning  inductance  until  resonance  is  obtained. 


STANDARD  MARINE  SETS  OF  AMERICAN  MARCONI  CO.  253 

When  connected  to  aerials  having  natural  wave  lengths  between  250  and  300 
meters,  these  sets  will  give  antenna  current  of  from  3  to  4  amperes,  which  will 
permit  communication  over  several  hundred  miles. 

202.  Aerial   Current  and   Reduction   of  Power. — In   order   to   familiarize 
operators  with  the  approximate  values  of  aerial  current  to  be  expected  from  the 
different  Marconi  transmitters  at  the  three  standard  waves,  the  following  table 
is  appended.     Different  values  will  be  obtained  with  different  aerials  but  those 
given  may  be  considered  as  a  good  average : 

Type  of  Transmitter.  300  meters.  450  meters.                  600  meters. 

2  K.  W.  500  cycle  3  to  5      amperes  9  to  13  amperes          12  to  17  amperes 

y^  K.  W.  500  cycle  2  to  4^  amperes  5  to    7  amperes          5^  to  8  amperes 

1  K.  W.    60  cycle  \y2  to  3  amperes  5  to  7      amperes 
Y2  K.  W.  120  cycle  \y2  to  3  amperes  5  to  7      amperes 

2  K.  W.  240  cycle  1  *%,  to  3  amperes  Sy2  to  8  amperes 
y4  K.  W.  500  cycle  1  to  2     amperes  3  to  4     amperes 

(a)  Reduction  of  Power.  The  International  Regulations  require  that  the 
transmitter  be  constructed  to  permit  the  power  to  be  reduced  progressively  from 
maximum  to  zero.  The  United  States  regulations  permit  the  power  to  be  reduced 
in  certain  types  of  sets  by  reduction  of  the  coupling  between  the  primary  and 
secondary  windings,  which  reduces  the  antenna  current. 

In  the  quenched  gap  sets,  the  power  is  reduced  ( 1 )  by  cutting  out  the  gaps, 
(2)  by  reduction  of  the  generator  voltage  or  (3)  by  lowering  the  coupling  if 
necessary.  The  power  of  the  synchronous  spark  sets  is  reduced  by  reduction  of 
the  coupling  between  the  primary  and  secondary  zvindings  of  the  oscillation  trans- 
former (thereby  reducing  the  antenna  current)  or  by  lowering  the  generator 
voltage. 

In  order  to  secure  a  uniform  spark  discharge  under  reduced  power,  it  may 
be  necessary  with  synchronous  spark  sets  to  slightly  readjust  the  position  of  the 
stationary  electrodes. 

Certain  types  of  transmitting  sets  are  provided  with  primary  or  secondary 
reactance  coils  which  reduce  the  primary  or  secondary  current  to  a  minimum 
value,  but  there  is  an  objection  to  reducing  the  power  in  this  way  because  a  total 
readjustment  of  the  gap  and  secondary  voltage  is  required  for  clear  tones. 

In  sets  like  the  Marconi  2  K.  W.  240  cycle  transmitter  or  the  1  K.  W.  60  cycle 
transmitter,  the  antenna  current  is  reduced  by  turning  the  secondary  winding  of 
the  type  A  oscillation  transformer  at  right  angles  to  the  primary  winding  or  at 
any  intermediate  position.  (See  curve  Fig.  245A). 

203.  General  Instructions  for  the  Panel  Sets. — Operators  in  the  Marconi 
service  are  urged  to  give  careful  attention  to  the  following  general  instructions 
for  the  care  and  maintenance  of  the  panel  transmitters : 

(1)  Observe  the  oil  containers  of  the  generator  and  motor  bearings  frequently 
and  keep  them  well  supplied  with  oil.     Open  small  petcock  occasionally 
for  test. 

(2)  Keep  close  watch  on  the  spring  contacts  of  the  aerial  changeover  switch 
for  looseness  of  contact. 

(3)  Wipe  the  insulating  rods  of  the  panel  board  frequently  with  a  slightly 
oiled  cloth  to  prevent  the  accumulation  of  dust  or  moisture. 

(4)  Make   daily  test  of  the   reading   of  the   aerial  ammeter.     Vary  the   in- 
ductance of  the  aerial  tuning  inductance  slightly  to  compensate  for  vary- 
ing sag  of  the  wires. 

(5)  Make  daily  test  of  the  auxiliary  battery  or  auxiliary  set.     Keep  batteries 
fully  charged. 

(6)  Tighten  up  all  nuts  and  bolts  on  the  panel  board  frequently. 

(7)  Take  up  the  adjusting  nut  on  the  quenched  gap  a  fraction  of  a  turn  now 
and  then. 

(8)  Keep  brushes  on  the  motor  clean. 


254  PRACTICAL   WIRELESS   TELEGRAPHY. 

(9)  Keep  close  watch  on  the  dead-end  switch  spring  contacts  of  the  receiving 
tuner.    Also  keep  metal  sectors  on  the  rubber  rod  and  the  brushes  clean. 

(10)  If  aerial  ammeter  burns  out,  disconnect  it  completely  from  the  thermo 
couple.    If  necessary  place  a  shunt  around  the  heater  terminals. 

(11)  If  the   resistance  coils  in  series  with  the   solenoid  windings  burn  out, 
substitute  two  16  C.  P.  lamps  connected  in  parallel.     Should  the  plunger 
on  the  automatic  starter  not  rise  when  the  starting  switch  is  closed,  raise 
it  by  hand  and  block  it  in  position  with  a  stick  of  wood,  for  temporary 
operation.     At  an  opportune  time  examine  the  resistance  coil  in  series 
with  the  solenoid  winding. 

(12)  Oil  bearings  and  gear  wheels  of  wave  length  and  coupling  adjustment 
with  a  few  drops  of  thin  oil. 

(13)  In  so   far  as  possible  protect  set  from  moisture  at  all   times.     When 
sparking  takes  place  at  the  safety  gap,  readjust  rotary  gap  or  use  less 
plates  at  the  quenched  gap. 

(14)  Do  not  under  any  circumstances  exceed  the  power  rating  of  the  set. 

(15)  For  broad  interfering  wave  use  close  coupling  and  full  power  input. 
Although  the  circuits  and  description  of  the  various  parts  of  a  complete  panel 

transmitter  when  treated  in  detail  may  appear  rather  complicated  to  the  beginner, 
he  should  bear  in  mind  that  these  senders  are  entirely  automatic  in  operation. 
In  fact,  during  practical  operation,  in  order  to  change  the  apparatus  from  a 
transmitting  to  a  receiving  position,  the  operator  is  only  required  to  shift  the 
position  of  the  antenna  changeover  switch. 

Moreover  with  the  panel  sets,  the  length  of  the  radiated  wave  can  be  instantly 
changed  from  one  to  the  other  of  the  three  standard  waves  (300,  450  and  600 
meters)  by  merely  throwing  a  switch. 

One  skilled  in  the  knowledge  of  telegraphy  could  learn  to  manipulate  a 
transmitter  and  receiver  within  a  day's  practice,  but  more  detailed  knowledge  of 
the  circuits  is  required  on  the  part  of  the  operator,  in  order  that  he  may  be  able 
to  cope  with  such  troubles  as  may  arise  at  sea. 


PART  XIII. 

MARCONI  DIRECTION  FINDER  OR  WIRE 

LESS  COMPASS  AND  ITS 

APPLICATION. 

204.  IN  GENERAL.  205.  DESCRIPTION  OF  EQUIPMENT.  206. 
THE  DIRECTION  FINDER  AERIALS.  207.  THE  CIRCUIT  COM- 
PLETE. 208.  THE  TUNED  BUZZER  TESTER.  209.  How  CUR- 
RENT Is  INDUCED  IN  THE  LOOPED  AERIALS.  210.  DIRECTION 
OF  MAGNETIC  FORCES  WITHIN  THE  GONIOMETER.  211.  GEN- 
ERAL INSTRUCTIONS  FOR  OPERATION  OF  THE  DIRECTION  FINDER. 
212.  To  FIND  THE  DIRECTION  OF  A  RADIO  STATION. 


204.  In   General. — The   radiogoniometer  or  direction  finder  is  a  specially 
designed  receiving  apparatus  for  determining  the  direction  of  a  wireless  tele- 
graph transmitting  station  at  a  given  receiving  station.    The  device  was  primarily 
intended  as  an  aid  to  navigation,  enabling  the  officer  of  a  vessel  to  make  observa- 
tions and  establish  his  position  independent  of  weather  conditions,  such  as  fog, 
etc.     It  is  applicable  in  many  other  ways  also  and  can  be  employed  to  advantage 
by  armies  and  navies ;  by  means  of  it  a  hostile  wireless  station  may  be  definitely 

located,  or  the  direction  of  the  enemy's 
battleships  while  in  radio  communica- 
tion, "sensed." 

Government  inspectors  are  likewise 
enabled  to  "round-up"  interfering  ama- 
teur stations  by  using  the  direction  finder. 
The  apparatus  is  even  of  considerable 
value  for  ordinary  receiving  purposes 
(short  range  work),  for  it  allows  the  re- 
ceiving operator,  when  the  ether  in  a 
given  locality  is  congested,  to  "screen  out" 
unwanted  wireless  telegraph  signals. 

The  Marconi  direction  finder  is  an 
adaptation  of  the  apparatus  originally 
evolved  by  Messrs.  Bellini  and  Tosi ; 
however,  the  device  as  produced  by 
these  inventors  was  not  adapted  to 
ship  work.  Improvements  were  made 
by  the  Marconi  Company  and  equip- 
ment is  now  turned  out  in  a  form  en- 
tirely satisfactory  for  use  by  naviga- 
tors, giving,  as  it  does,  a  high  degree 
of  accuracy. 

205.  Description  of  Equipment. — The  complete  equipment  consists  of  a 
goniometer  with  the  necessary  appliances  for  control  as  in  Fig.  267,  a  tuned  imre- 
less  telegraph  receiver  shown  in  Fig.  268,  a  tuned  buzzer  tester  as  in  Fig.  269, 
and  an  angle  divider  as  in  Fig.  270. 


Fig.  267 — The   Radio   Goniometer. 


256 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Fig.    268 — The    Tuned    Receiver   for   the    Direction 
Finder. 


206.  The  Direction  Finder  Aerials.— A  distinctive  feature  of  the  direction 
finder  equipment  is  the  use  of  two  closed  circuit  looped  aerials  having  the  form 
of  an  isosceles  triangle  as  shown  in  Fig.  271.  These  aerials  bisect  each  othfer  at 

right  angles  and  also  hold  an  angle 
of  45°  with  the  bow  and  stern  line 
of  the  vessel. 

It  is  important  that  the  aerials  be 
placed  in  a  somewhat  clear  space  on  the 
deck  and  that  the  two  loops  have  identical 
dimensions.  The  wires  should  be  held 
taut  and  firmly  in  place.  The  current  col- 
lected by  the  aerials,  from  the  advancing 
electromagnetic  waves,  is  made  to  flow 
through  a  specially  designed  set  of  excita- 
tion coils,  setting  up  a  magnetic  field  which 
acts  upon  a  third  coil  known  as  the  ex- 
ploring coil.  The  latter  coil  carries  a 
pointer  which  moves  over  a  360°  scale  and 
gives  the  sense  of  direction  of  any  trans- 
mitting station.  This  portion  of  the  ap- 
paratus is  known  as  the  goniometer,  the 
windings  for  which  are  clearly  shown  in 
the  photograph,  Fig.  272.  It  will  be  re- 
ferred to  again  hereafter. 
The  sketch,  Fig.  273,  is  a  plan  view  of  the  two  triangular  aerials  as  previously  described, 
showing  their  relative  positions  to  the  bow  and  stern  line  of  a  given  vessel  (marked  B 
and  S). 

Before  entering  into  a  detailed  explanation  of  the  circuits,  the  diagram  in  Fig.  274 
should  have  consideration.  Let  A  and  B  represent  two  sides  of  a  single  loop  of  the  direc- 
tion finder  aerial,  and  the  arrows,  the  direction  of  the  flux  in  a  passing  wireless  wave ; 
then,  this  loop  receives  the  maximum  induction  when  its  plane  bears  the  position,  to  the 
passing  wave,  shown.  Furthermore,  a  little  study  of  the  diagram  will  show  that  the  current 

set  up  by  this  flux  in  side  A,  is  in  op- 
position to  that  set  up  in  side  B ;  but  if 
the  two  sides  of  the  loop  are  spaced  a 
certain  distance  apart,  the  current  in- 
duced in  side  B  will  attain  maximum 
amplitude  an  instant  later  than  in  side 
A;  hence,  an  electric  current,  the  re- 
sult of  the  two  E.  M.  F.'s  flows  around 
the  loop  A,  B.  The  magnetic  flux 
generated  thereby,  in  the  coil  L,  acts 
inductively  upon  the  exploring  coil  L-2 
the  resulting  oscillations  being  rectified 
by  the  detector  and  associated  appa- 
ratus, and  made  audible  in  head  tele- 
phones in  the  usual  manner. 

It  is  plainly  evident  that  both  sides 


Fig.  270 — Angle  Divider. 


Fig.  269 — Tuned  Buzzer  Tester. 


DIRECTION   FINDER  OR  WIRELESS  COMPASS. 


257 


of  the  loop  A,  B,  are  acted  upon  equally  and  at  the  same  instant  when  its  plane  is  at  a  right 
angle  to  the  plane  of  the  advancing  wave.  Equal  and  opposing  electromotive  forces  will,  there- 
fore, be  induced  in  both  legs,  resulting  in  no  flow  of  current. 

If,  however,  the  loop  A,  B,  is  acted  upon  at  any  other  angle  than  a  right  angle,  an  elec- 
tromotive force  will  be 
induced,  the  intensity  of 
which  varies  as  the  co- 
sine of  the  angle  which 
the  advancing  flux 
makes  with  the  loop. 

The  description  just 
given  does  not  take  into 
account  the  phenomena 
involved  when  both 
loops  are  employed  and 
the  consequent  effect  on 
the  goniometer  coil.  This 
will  be  explained  in  a 
later  paragraph.  We 
shall  first  proceed  to  a 
description  of  the  direc- 
tion finder  complete. 

207.  The  Circuit 
Complete.  —  Careful 
inspection  should  be 
made  of  the  diagram  of 
connections  in  Fig.  275. 
The  triangular  loop 
aerial,  A,  B,  is  connected 
in  series  with  the  vari- 
able condenser  K  and  to 
the  excitation  coil  L. 
The  loop  aerial  C,  D,  is 
connected  in  series  with  condenser  K-l  and  the  excitation  coil  L-l.  The  condensers  K  and 
K-l  have  identical  values  of  capacity  and  their  capacities  are  altered  simultaneously  by  a 
handle  mounted  on  the  top  of  the  box  as  shown  in  Fig.  267. 


TO  RtCElVER 


Fig.     271 — The     Triangular     Aerial     Employed 
Direction    Finder. 


in     Connection 


the 


Fig.  272 — Detail  of  the  Radio   Goniometer. 


Fig.    273— Diagram    Showing   the    General    Layout    of 
the   Direction    Finder   Aerials. 


258 


PRACTICAL  WIRELESS  TELEGRAPHY. 


The  magnetic  field  produced  by  the  coils  L  and  L-l  combine  and  act  upon  the  exploring 
coil  L-2,  the  position  of  which  will  be  more  clearly  understood  from  the  photograph,  Fig. 
272,  which  also  shows  the  rectangular  coils  to  be  connected  to  the  loops  of  the  aerial.  The  ex- 
ploring coil  is  placed  inside  the  antenna  coils  and  is  turned  by  the  handle  on  the  top  of  the  set. 
The  oscillations  induced  in  L-2  by  the  oscillatory  current  flowing  in  the  coils  L  and  L-l  is 
transferred  to  the  inductance  coil  of  the  local  detector  circuit  L-4  by  the  coil  L-3. 

The  coil  L-2,  the  variable  condenser 
V,  and1  the  coil  L-3,  constitute  an  inter- 
mediate circuit  similar  to  that  employed 
in  the  well-known  Marconi  valve  tuner. 
The  coupling  between  the  coils  L-3  and 
L-4  is  varied  by  a  knob  mounted  on  the 
side  of  the  tuned  receiver. 

The  detector  circuit  consists  of  the  in- 
ductance coil  (of  fixed  value)  L-4;  the 
billi-condenser  V-l  in  shunt;  the  fixed 
condenser  V-2;  the  head  telephones  H; 
the  potentiometer  P  and  the  battery  B. 
Two  detectors  are  employed  for  this  set; 
one  a  crystal  of  carborundum,  the  other 
a  crystal  of  cerusite;  either  one  of  which 
may  be  connected  in  the  circuit  as  de- 
sired. When  the  cerusite  detector  is  in 
use,  the  battery  and  potentiometer  are  cut 
out  of  the  circuit. 

208.  The  Tuned  Buzzer  Tester.— 

The  direction  finder  equipment  also  in- 
cludes a  tuned  buzzer  tester  which  may 
be  set  to  radiate  waves  either  300  or  600 
meters  in  length.  The  buzzer  box  has 
four  holes,  one  in  each  corner,  through 
which  the  four  leads  from  the  two  looped 
aerials  pass.  These  holes  are  marked  S  F 
(Starboard  Forward},  S  A  (Starboard 
Aft},  P  A  (Port  Aft}  and  P  F  (Port 
Forward}.  Care  should  be  taken  to  bring 
the  corresponding  leads  from  the  loop 
aerials  through  the  proper  holes.  The 
leads  for  both  aerials  are  now  in  inductive 
relation  to  the  tuned  buzzer  circuit ;  hence, 
the  aerials  and  in  fact  the  entire  apparatus  may  be  preadjusted  to  the  two  standard  wave 
lengths,  assuring  that  a  sender  transmitting  on  one  of  these  waves  will  be  heard. 

It  is  sufficient  for  the  time  being  to  say  that  for  the  practical  operation  of  the  apparatus, 
the  aerial  circuit,  the  intermediate  circuit,  and  the  local  detector  circuit  must  be  accurately 
tuned  to  resonance  and  to  the  incoming  waves.  The  exploring  coil  is  then  turned  on  its 
axis  until  the  maximum  strength  of  signals  is  secured  in  the  head  telephones.  The  pointer 
on  the  coil  L-2  will  then  lie  in  the  direction  of  the  transmitting  station. 

We  shall  now  explain  the  induction  of  current  in  the  two  looped  aerials  by  an  advancing 
electromagnetic  wave. 

209.  How  Current  is  Induced  in  the  Looped  Aerials. — If  the  waves  from 
a  given  transmitting  station,  Fig.  273,  advance  in  the  general  direction  E,  oscillating  cur- 
rents will  be  induced  in  the  loop  C,  D,  but  none  in  loop  A,  B.  To  obtain  the  maximum 
induction  from  these  oscillations,  the  exploring  coil  must,  in  this  case,  lie  parallel  to  the 
goniometer  excitation  winding  connected  to  the  terminals  of  the  loop  C,  D. 

Again,  if  the  waves  advance  in  the  direction  E-l,  the  induction  on  the  loop  A,  B  is  maxi- 
mum and  on  C,  D  nil.  Hence,  the  exploring  coil  must  now  lie  parallel  to  the  goniometer 
winding  connected  to  the  loop  A,  B  to  receive  the  maximum  induction,  and  the  pointer  will 
lie  along  the  direction  E-l. 

If  the  waves  advance  in  the  direction  E-2,  both  loops  are  acted  upon  simultaneously  and 


vOOQOQ 


a     Single     Loop 


DIRECTION  FINDER  OR  WIRELESS  COMPASS. 


259 


AERIALS 


TUNED   BUZZER  CIRCUIT 


CARBORUNDUM 

Fig.    275 — Complete   Wiring   Diagram    of   the    Marconi    Direction    Finder, 


260  PRACTICAL  WIRELESS  TELEGRAPHY. 

equally,  and  an  oscillatory  current  flows  through  both  excitation  windings  of  the  goniometer. 
The  magnetic  fields  set  up  by  the  two  coils  combine  and  produce  a  resultant  Held  from 
which  the  maximum  induction  will  be  obtained  in  the  detector  circuit,  when  the  pointer  of 
the  exploring  coil  lies  parallel  to  a  line  drawn  midway  between  C  and  D,  A  and  B  (dotted 
line). 

But  let  the  waves  arrive  in  the  direction  E-3 ;  in  this  case  the  loop  A,  B  will  receive  the 
maximum  induction  of  the  advancing  wave  while  the  loop  C,  D  will  be  acted  upon  feebly. 
To  obtain  the  maximum  induction  from  the  resulting  magnetic  field,  the  goniometer  coil 
will  be  in  such  position  that  the  pointer  will  lie  along  the  direction  E-3. 

210.  Direction  of  Magnetic  Forces  Within  the  Goniometer. — Figs.  276  and 
277  outline  the  production  of  the  magnetic  field  within  the  goniometer  when  both  aerials 
are  acted  upon  by  the  advancing  wave  simultaneously.     It  should  be  remembered  that  in 
order  to  induce  the  maximum  current  in  the  detector  circuit,  the  exploring  coil  is  always 
at  right  angles  to  the  magnetic  field  within  the  goniometer  windings.     This  is  shown  in 
the  diagram,  Fig.  276,  which  is  in  fact  a  plan  view  looking  down  from  the  top  on  a  single 
coil  connected  to  one  of  the  loop  aerials.    The  direction  of  the  magnetic  lines  of  force  with 
a  given  direction  of  current  and  the  corresponding  position  of  the  exploring  coil  for  maxi- 
mum induction  is  clearly  mapped  out  as  well  as  the  position  of  the  pointer  on  the  scale  and 
its  mounting  relative  to  the  coils. 

Fig.  277  is  a  plan  view  looking  down  from  the  top  on  the  two  coils  of  the  goniometer 
showing,  by  the  dotted  lines,  the  resultant  magnetic  field  when  both  aerials  are  acted  upon 
simultaneously  and  with  equal  intensity.  If  current  flows  in  the  coil  A,  B  only  and  in  the 
direction  shown,  the  corresponding  magnetic  lines  of  force  will  take  the  general  direction 
of  the  heavy  lines  (the  N  and  S  polarity  being  as  indicated).  And  again,  if  only  the  loop 
C,  D  is  acted  upon  by  the  advancing  waves,  then  the  general  direction  of  flux  in  the 
corresponding  goniometer  coil  will  be  that  shown.  But  when  current  Hows  in  both  coils  at 
the  same  time,  it  is  clearly  seen  that  the  corresponding  fluxes  are  at  right  angles  and  a 
resultant  field  is  produced  which  will  take  the  paths  F  and  F-\  and  the  pointer  on  the  coil, 
for  maximum  induction,  will  lie  in  the  direction  P  and  P-l.  The  exploring  coil  must,  there- 
fore, be  in  the  position  shown  to  receive  the  maximum  induction  from  the  resultant  mag- 
netic field. 

211.  General  Instructions  for  Operation  of  the  Direction  Finder. — (1)  As 

a  matter  of  convenience,  the  box  containing  the  goniometer  coils  and  the  variable  condenser, 
should  be  so  placed  that  the  zero  position  on  the  scale  coincides  with  the  bow  and  stern  line 
of  the  vessel. 

(2)  The  tuned  buzzer  circuit  is  then  set  into  operation  at  either  300  or  600  meters,  de- 
pending upon  which  wave-length  it  is  desired  to  receive. 

(3)  The  coupling  knob  on  the  tuned  receiver  is  turned  to  90°. 

(4)  With    the   buzzer   in   operation,    the   condenser   connected1  across    the    intermediate 
circuit  and   the  billi-condenser  are  altered   in  capacity   (simultaneously  adjusting  the  de- 
tector) until  maximum  response  is  secured  in  the  head  telephones. 

(5)  When  the  foregoing  adjustments  have  been  made,  the  capacity  of  the  condenser  in 
the  antenna  circuit  is  altered  by  the  knob  on  the  top  of  the  goniometer  box  until  a  still 
greater  response  in   the  head  telephones  is  obtained.     It   may  then  become  necessary  to 
slightly  readjust  the  values  of  capacity  in  use  at  the  intermediate  circuit  condenser  and  the 
billi-condenser. 

(6)  When  the  two  loop  aerials  are  in  use  and  the  buzzer  is  in  operation,  the  loudest 
signals  are  obtained   from  the  buzzer  when  the   pointer  is  at  zero.     The   signals   should 
gradually  decrease  in  strength  as  the  pointer  is  moved  toward  the  90°  position. 

(7)  When  the  two  aerials  are  in  use,  zero  signals  should  be  obtained  when  the  pointer 
is  in  the  position  90°-90e. 

(8)  When  one  of  the  aerial  loops  is  disconnected  by  means  of  the  switch  on  the  top  of 
the  goniometer  box,  the  maximum  signal  is  received  with  the  pointer  at  45°-135°  in  one 
direction  and  the  zero  signal  with  the  pointer  45°-135°  in  the  opposite  direction.     The  re- 
verse condition  takes  place  when  the  second  loop  is  in  use  alone. 

(9)  If,  when  the  loops  are  thus  tested  singly,  it  becomes  necessary  to  alter  the  capacity  of 
the  intermediate  or  the  billi-condenser  for  maximum  response,  it  is  a  positive  indication 
that  the  loops  are  unsymmetrical  and  therefore  out  of  balance.    Identical  positions  of  con- 
denser capacity  should  be  obtained  on  both  loops.     Steps  should  be  taken  immediately  to 


DIRECTION   FINDER  OR  WIRELESS  COMPASS. 
FLUA 


261 


Fig.    276 — Showing    the    Magnetic    Flux    Within    a    Single    Coil    of    the 
Goniometer    Under    Given    Conditions. 


TO  AERIAL 


EXCITATION  COIL 


DIRECTION  OF 
ADVANCING 
WAVE 


& 

TO  AERIAL 


Fig.  277— Showing  the  Resultant  Flux  Set  Up  in  the  Two  Excitation  Coils 
of   the    Goniometer. 


262  PRACTICAL  WIRELESS   TELEGRAPHY. 

correct  this  error  which  undoubtedly  lies  in  the  aerial.  A  slight  compensation  for  this  un- 
symmetrical  condition  may  be  made  at  the  variable  condensers  insid'e  the  box  by  means  of 
two  small  adjusting  screws. 

(10)  The 'strength  of  the  signals  in  one  loop  should  be  identical  with  that  in  the  other. 
If  not,  a  bad  connection  probably  exists  in  one  of  the  aerials. 

212.  To  Find  the  Directions  of  a  Radio  Station. — (1)  The  station  whose 
direction  is  to  be  determined  if  not  already  in  the  act  of  sending,  should  be  called  and 
requested  to  send  a  test  letter  for  two  or  three  minutes,  making  sure  to  disconnect  the  two 
looped  aerials  of  the  direction  finder  by  switches  mounted  on  the  top  of  the  goniometer 
box  during  the  sending  period. 

(2)  When  a  reply  is  received  (on  the  ordinary  receiving  equipment),  the  ship's  trans- 
mitter and  receiver  are  put  out  of  action,  even  to  the  disconnecting  of  the  flat  top  aerial 
from  the  earth,  which,  if  left  in  the  circuit,  will  seriously  affect  the  accuracy  of  the  goni- 
ometer reading. 

(3)  Next  close  the  two  switches  for  the  loop  aerials  and  swing  the  direction  finder 
handle  until  the  maximum  strength  of  signals  is  obtained  in  the  head  telephone.    This  should 
not  be  a  difficult  adjustment,  as  the  apparatus  has  prevoiusly  been  adjusted  to  the  maximum 
degree  of  sensitiveness  and  to  the  standard  wave  length  adjustment  by  means  of  the  test 
buzzer.    The  pointer  will  now  indicate  on  the  goniometer  scale  the  general  direction  of  the 
transmitting  station  from  which  signals  are  being   received,   that  is  to  say,  it  shows  the 
direction  with  respect  to  the  bow  and  stern  line  of  the  vessel. 

(4)  If  the  signals  received  are  not  sharply  denned,  having  about  equal  strength  over 
a  considerable  portion  of   the  goniometer   scale,   the  positions   should  be  noted  where  the 
signals  fall  to  zero   (above  and  below  maximum)   and  a  mean  of  the  two  readings  taken. 
The  mean  reading  is  obtained  by  the  angle  divider  furnished  with  the  set. 

(5)  Before  transmission  is  again  resumed,  care  should  be  taken  to  disconnect  the  two 
loop  aerials  from  the  goniometer  by  means  of  the  switches  previously  described,  and  also 
to  put  the  detector  switch  at  zero. 

It  should  be  understood  that  the  direction  finder  only  gives  the  sense  of  direc- 
tion in  reference  to  the  bow  and  stern  line  of  a  vessel  and  not  the  geographical 
direction  of  the  wireless  station,  which,  of  course,  must  be  obtained  by  the  readings 
of  the  standard  ship's  compass.  More  clearly,  the  direction  finder  gives  the  angle 
which  the  advancing  wave  from  the  transmitting  station  makes  with  the  center 
line  of  the  vessel. 

For  instance,  if  the  pointer  of  the  direction  finder  indicates  that  ±he  general  direction 
of  the  transmitting  station  is  20°  off  the  port  bow,  it  does  not  show  whether  that  station 
is  20°  to  the  port  bow  or  20°  to  the  starboard  quarter.  In  the  case  of  land  stations  there 
need  be  no  doubt  concerning  this,  as  it  is  generally  known  whether  the  station  is  to  the 
port  or  starboard  side  of  the  vessel.  There  can  never  be  much  doubt  as  to  whether  a  ship 
is  approaching  or  receding  from  a  land  station,  for  by  the  reverse  interpretation,  the  land 
station  would  be  located  at  sea — an  obvious  absurdity. 

//,  in  case  of  a  heavy  fog,  the  signals  from  another  ship  indicated  that  it  bore  a  direc- 
tion over  the  bow  and  stern  line  of  the  vessel,  and  the  signals  from  the  ship  became  o/l 
gradually  increasing  intensity,  it,  of  course,  indicates  that  the  ships  are  approaching  each 
other,  but  does  not  show  whether  bow-on,  or  in  the  same  general  direction.  A  wireless 
message  sent  to  the  ship  asking  her  course,  would  of  course,  remove  all  doubt  and  enable 
the  navigator  to  avoid  a  collision. 

Readings  may  be  taken  simultaneously  of  two  land  stations,  and  the  position  of  the 
vessel  located  by  well  known  navigation  methods.  Readings  may  be  taken  from  a  single 
station  and1  the  ship  moved  forward  in  a  straight  course  to  a  definite  distance  and  a  second 
observation  made.  The  data  obtained  in  this  manner  is  sufficient  to  establish  the  position  of 
the  vessel. 

An  interesting  application  of  the  direction  finder  is  the  assistance  it  gives  in  locating  a 
distressed  vessel  in  foggy  weather  or  after  darkness.  After  the  distress  signals  have  been 
received1,  the  position  of  the  distressed  vessel  transmitted  and  certain  information  given  to 
the  relief  vessel  to  enable  the  commander  to  know  what  direction  he  shall  take,  the  distressed 
vessel  is  then  asked  to  make  the  test  letter  "V",  or  any  other  prearranged  signal.  The  direc- 
tion finder  of  the  relief  vessel  is  then  put  into  operation  and  the  general  direction  of  the 
signals  of  the  distressed  vessel  obtained. 


DIRECTION  FINDER  OR  WIRELESS  COMPASS. 


263 


The  relief  vessel  is  then  swung  by  the  helm  until  the  bow  and  stern  line  of  the  vessel 
coincide  with  the  position  of  the  pointer  on  the  direction  finder  where  maximum  signals  are 
obtained.  In  this  manner,  the  relief  vessel  can  be  kept  on  a  direct  course  and  the  distressed 
vessel  located  in  the  quickest  possible  manner. 

When  entering  ports  like  New  York  harbor,  where  the  atmosphere  is  at  times  congested 
with  wireless  traffic,  the  direction  finder  has  been  employed  in  a  most  efficient  manner  for 
eliminating  unwanted  signals.  First  one  loop  is  thrown  in  the  circuit;  if  the  station  desired 
is  not  heard,  the  second  loop  is  thrown  in  and  a  test  made.  If  the  signals  are  received  on 
this  loop,  50  per  cent,  of  the  local  interference  under  some  conditions  may  be  wholly  elimi- 
nated, or  at  least  detuned  to  such  an  extent  as  to  be  negligible.  Or,  if  desired,  both  loops 
of  the  direction  finder  may  be  employed,  and  the  pointer  of  the  goniometer  set  in  the  direc- 
tion of  the  station  from  which  signals  are  being  received.  It  is  certain  that  in  this  direction 
the  maximum  strength  of  signals  will  be  received,  and  thos.e  of  all  other  stations  not  on  the 
same  general  line  will  be  reduced  or  wholly  excluded. 

Under  favorable  conditions,  bearings  may  be  taken  with  the  direction  finder  within  two 
or  three  degrees  of  accuracy ;  the  error  due  to  the  instrument  itself  does  not  exceed  1  degree. 
The  range  of  this  apparatus  with  a  carborundum  detector  is  from  40  to  50  miles,  but  with  a 
cerusite  detector  it  may  be  extended  to  160  or  170  mile.s,  which  is  ample  for  navigation 
purposes. 

The  direction  finder  has  been  found  applicable  to  the  reception  of  long  wave  length 
signals  over  great  distances.  Recent  tests  with  this  aerial  were  carried  out  by  the  Marconi 
Wireless  Telegraph  Company  of  America,  at  their  transatlantic  receiving  station,  located 
at  Belmar,  N.  J.,  U.  S.  A.  The  direction  finder  loop  aerials  were  erected  on  a  450-foot 
mast,  and  the  receiving  apparatus  designed  to  be  responsive  to  waves  inclusive  of  10,000 
meters.  With  a  receiver  responsive  to  undamped  oscillations,  daylight  signals  were  re- 
ceived from  the  Marconi  Stations  at  Carnarvon,  Wales,  and  Clifden,  Ireland;  also  from 
the  German  stations  at  Nauen,  Germany,  and  Hanover,  Germany.  The  direction  of  these 
stations  was  readily  found  with  a  notable  degree  of  accuracy. 


Fig.    277a — The    Direction    Finder    Complete    with    Fleming    Oscillation    Valve. 


\ 

PART  XIV. 

TRANSMITTERS  OF  UNDAMPED 
OSCILLATIONS. 

ARC  GENERATORS— RADIO-FREQUENCY  ALTERNATORS— 
PLIOTRON  OSCILLATOR. 

213.  IN  GENERAL.  214.  THE  ARC  GENERATOR.  215.  SIGNAL- 
LING WITH  THE  ARC  TRANSMITTER.  216.  THE  ALEXANDERSON 
HIGH  FREQUENCY  ALTERNATOR.  217.  GOLDSCHMIDT  RADIO- 
FREQUENCY  ALTERNATOR.  218.  THE  JOLY  SYSTEM  FOR  THE 
PROTECTION  OF  UNDAMPED  OSCILLATIONS.  219.  MARCONI'S 
SYSTEM  FOR  THE  PRODUCTION  OF  CONTINUOUS  WAVES. 
220.  THE  PLIOTRON  OSCILLATOR. 

213.  In  General. — Progress  in  the  United  States  in  the  development  of 
sustained  oscillation  generators  has  been  particularly  marked  during  the  past  few 
years. 

In  fact,  by  the  use  of  undamped  wave  apparatus  the  transmitting  range  of 
high-power  stations  has  been  materially  increased;  and  in  view  of  the  wide- 
spread interest  in  continuous  wave  equipment,  the  fundamental  circuits 
and  operation  of  some  of  the  more  important  systems  will  be  briefly  described 
and  explained. 

Sustained  waves  are  defined  as  the  waves  radiated  from  a  conductor  in  which  alternating 
current  flows.  The  terms  "undamped"  "sustained"  and  "continuous"  are  often  employed 
synonymously  and  are  presumed  to  indicate  oscillations  of  continuous  amplitude  and  constant 
formation  in  contrast  to  "discontinuous"  oscillations  which  occur  in  groups  of  200  to  1,000  per 
second,  and  are  obtained  by  the  recurring  charge  and  discharge  of  a  condenser  across  a 
spark  gap.  A  slight  distinction  in  the  use  of  these  terms  should  be  made,  as  electrical 
oscillations  may  be  continuous  in  the  sense  that  they  are  not  broken  up  into  distinct  groups 
but  the  successive  cycles  may  vary  in  amplitude,  an  example  being  the  type  of  oscillations 
generated  by  the  arc. 

Continuous  waves  are  set  into  motion  by: 

(1)  The  radio-frequency  alternator  such  as  the  Alexanderson  and  Goldschmidt 
types. 

(2)  The  Poulsen  or  Duddell  arc  generator. 

(3)  A  battery  of  vacuum  valve  tube,  such  as  the  General  Electric  Pliotron  oscil- 
lators. 

At  the  writing  of  this  volume,  marine  wireless  telegraph  sets  are  fitted  with  discontinuous 
wave  transmitters,  almost  exclusively,  but  high  power  stations  employ  either  continuous  or 
discontinuous  wave  apparatus.  One  reason  why  continuous  wave  transmitters  have  not  been 
universally  adopted  for  ship  service  is  accounted  for  by  the  fact  that  the  apparatus  for  the 
production  of  continuous  oscillations  (or  waves)  generally  involves  mechanical  and  electrical 
problems  difficult  to  surmount  or  else  the  apparatus  is  too  intricate  or  cumbersome  to  be 
installated  aboard  ship.  It  is  therefore  safe  to  predict  that  discontinuous  wave  apparatus  will 
be  employed  in  this  branch  of  radio  communication  for  some  time  to  come. 


TRANSMITTERS  OF  UNDAMPED  OSCILLATIONS. 


265 


214.  The  Arc  Generator. — A  simple  system  for  generating  continuous 
oscillations  is  the  arc  generator,  the  circuits  of  which  are  shown  in  Fig.  278, 
where  an  arc  gap  of  copper  and  carbon  electrodes  A,  B,  is  shunted  to  the  terminals 
of  a  500  volt  direct  current  dynamo.  In  this  diagram  C-l  and  C-2  are  iron  core 
inductances  which  prevent  the  oscillations  of  radio-frequency  discharging  back 

into  the  generator.  C 
is  the  usual  con- 
denser of  the  closed 
oscillation  circuit 
and  L  the  variable 
inductance.  In  the 
usual  manner,  the 
oscillations  generat- 
ed in  this  circuit  are 
transferred  to  the 
antenna  circuit  by 
means  of  an  oscilla- 
tion transformer. 

The  functioning  of 
this  apparatus  is  as 
follows  :  When  the 

electrodes  of  the  arc  A,  B,  are  first  placed  in  contact  and  then  given  the  correct  separation, 
considerable  difference  of  potential  exists  across  the  terminals  as  can  be  shown  by  a  volt- 
meter. Accordingly  a  certain  amount  of  current  flows  into  the  condenser  C  which  receives 
a  charge.  The  charge  taken  by  the  condensers  robs  the  arc  of  some  of  its  current,  which 


JIM; 


iron 


Fig.    278— Fundamental    Diagram   of   the   Poulsen    Arc   Generator. 


(oooiooooo/ 


Fig.    279 — Complete    Circuits    of   a    Modern1    Arc   Transmitter. 

further  increases  the  potential  across  the  arc  and  places  a  still  greater  charge  on  the  con- 
denser. When  fully  charged,  the  condenser  discharges  across  the  arc  gap  and  lowers  the 
arc  voltage.  This  decrease  of  arc  voltage  also  assists  the  discharge  of  the  condenser  mate- 


266 


PRACTICAL  WIRELESS  TELEGRAPHY. 


rially  and  due  to  the  inertia  of  the  oscillation  circuit,  the  condenser  will  continue  to  discharge 
in  the  opposite  direction,  or,  in  other  words,  an  alternating  current  will  flow  through  the 
oscillation  circuit.  Due  to  the  fact  that  direct  current  is  constantly  supplied  to  the  arc  and 
condenser,  this  process  of  charge  and  discharge  continues  so  long  as  the  arc  is  in  operation, 
and  the  resultant  frequency  of  oscillation  is  approximately  that  due  to  the  inductance, 
capacity,  and  resistance  of  the  condenser  circuit.  The  oscillations  flowing  in  this  circuit 
are  transferred  to  the  aerial  wires  from  which  part  of  their  energy  is  radiated  in  the 
form  of  continuous  waves. 

It  is  found  that  enclosing  the  arc  gap  in  an  airtight  chamber  and  feeding  it  with  hydrogen 
gas,  alcoholic  vapors  or  even  steam  tends  to  increase  the  potential  difference  as  well  as  to 
prevent  the  arc  blowing  out.  It  has  been  determined  further  that  powerful  electromagnets 
mounted  at  a  right  angle  to  the  arc,  tend  to  increase  the  potential  difference  and,  at  the 
same  time,  cause  the  arc  electrodes  to  burn  more  evenly.  For  continuous  operation,  and 
to  prevent  overheating,  the  containing  chamber  and  the  arc  electrodes  must  be  cooled  by 
water  circulation. 

In  the  latest  naval  installations,  the  arc  gap  is  connected  directly  in  series 
with  the  antenna  circuit  as  shown  in  Fig.  279,  which  is  a  duplicate  of  the  con- 
nections of  the  U.  S.  Naval  Station  at  Radio,  Va.,  and  other  naval  stations.  It 

is   claimed   that   con- 

Y<  MO  VOLT5  f  j^>  necting  the  arc  in  this 

DC.  L__aX^  i»  way  has  mcreased  the 
j  range  of  such  trans- 

f  mitters  to  a  remark- 

(  "|  KEY  able  degree,  and  that 

-  the  operation  of  the 

system  as  a  whole  has 
been  simplified. 

Referring  to  the  dia- 
gram (Fig.  279)  :  The 
choking  inductances  are 
indicated  at  C-l  and  C-2  ; 
the  regulating  resistance 
for  the  arc  shown  at  R 
is  connected  in  series 
with  the  blow-out  mag- 
nets placed  at  right  angles 
to  the  arc.  A  source  of 
direct  current  of  500 
volts  is  connected  to  the 
arc  gap,  the  plus  pole  of 
which  is  constructed  of 
copper,  the  minus  pole  of 
carbon. 

An  arc  transmitter  of 
this  type  is  adjusted  for 
operation  as  follows  : 
The  arc  is  struck  at  low 
voltage  by  connecting 
large  values  of  resist- 
ance in  the  circuit  at  R. 
The  gap  is  then  length- 
ened out  and  the  voltage 
increased  until  the  aerial 

'  reading.*  /*  is 

noted     that     a     critical 

length  of  arc  gap  gives  the  maximum  value  of  antenna  current  and  this  is  the  adjustment 
always  to  be  aimed  for. 

The  condenser  shunted  across  the  arc  (Fig.  279)  acts  as  a  by-pass  for  the  radio-frequent 
currents  flowing  in  the  antenna  circuit. 


Fig.  280-Details   and   Connections 


Akxanderson   Radio   Frequency 


TRANSMITTERS  OF  UNDAMPED   OSCILLATIONS. 


267 


215.  Signalling  with  the  Arc  Transmitter. — It  is  obvious  that  a  telegraph 
key  cannot  be  placed  in  series  with  the  arc  gap  for  signalling  and,  in  consequence, 
the  formation  of  the  Morse  characters  is  usually  effected  by  changing  the  in- 
ductance of  the  antenna  circuit  at  coil  L-l,  Fig.  279.  When  the  lever  of  the  key 
K  is  released,  the  radiated  wave,  for  example,  is  6,900  meters  in  length,  but  upon 
pressing  the  key,  a  few  additional  turns  of  wire  are  inserted  in  the  antenna  circuit ; 
the  wave  length  thus  is  increased  to  7,000  meters.  Now  if  the  receiver  is  tuned 
to  the  wave  radiated  by  the  transmitter  when  the  key  is  closed  (7,000  meters),  the 
wave  radiated,  when  the  key  is  open  (or  when  the  key  lever  is  up),  will  not  be 
heard  in  the  receiving  telephone,  hence  the  Morse  characters  are  formed  at 
the  sender  and  easily  distinguished  at  the  receiver. 

The  wave  radiated  when  the  key  is  released  is  termed  the  "compensation"  wave,  but  the 
second  wave  is  termed  the  "signalling  zvave."  In  stations  employing  this  system  of  signalling, 
it  is  difficult  to  distinguish  the  signals  of  the  sending  wave  unless  very  careful  resonant 
adjustments  are  made  at  the  receiver,  otherwise  a  mixture  of  the  two  radiated  waves  is 
obtained,  which  cannot  be  deciphered. 

An  important  feature  of  the  arc  system  is  the  ease  by  which  the  length  of  the  radiated 


100,000  CYCLE  ALTERNATOR 
SPEED    20,000  R.RM. 


DIRECT  CURRENT  MOTOR 
1IOV.OLT    2000  R.F.M. 


Fig.    280a— The    2    K.    W.    100,000    Cycle    Alexanderson    Alternator. 

wave  can  be  changed.  This  can  be  instantly  effected  by  simply  adding  or  subtracting  turns 
at  the  aerial  tuning  inductance. 

Undamped  wave  transmitters  of  the  arc  type  operate  most  efficiently  at  wave  lengths  in 
excess  of  3,000  meters,  and,  in  fact,  the  majority  of  arc  stations  operate  at  wave  lengths  in 
excess  of  6,000  meters.  Satisfactory  tests  have  been  carried  out  between  the  Darien 
Isthmus  of  Panama  and  San  Diego,  Cal.,  naval  stations  with  arc  sets  at  the  wave  length  of 
18,000  meters. 

Arc  generators  of  100  kilowatts  capacity  are  in  daily  use.  The  radio  station  at  Tucker- 
ton,  New  Jersey,  under  U.  S.  Navy  control,  employs  a  TOO  K.  W.  arc  set  which,  at  the  wave 
length  of  7,400  meters,  gives  antenna  current  of  150  amperes.  Communication  is  effected 
with  the  corresponding  station  at  Hanover,  Germany,  during  the  favorable  hours  of  the  day. 

The  U.  S.  Naval  Station  at  San  Diego,  Cal.,  is  fitted  with  a  200  K.  W.  arc  transmitter 
giving  an  antenna  current  of  120  amperes;  other  naval  stations  are  fitted  with  30  K.  W. 
and  60  K.  W.  transmitters.  The  naval  station  under  construction  at  Cavite,  Philippine 
Islands,  will  be  fitted  with  a  350  kilowatt  arc  set  giving  an  antenna  current  of  200  amperes. 

Daylight  communication  between  Radio,  Va.,  and  Darien  Isthmus  of  Panama,  and  San 
Diego,  Calif.,  has  been  carried  on  for  a  number  of  months. 


268 


PRACTICAL  WIRELESS  TELEGRAPHY. 


Certain  battleships  in  the  U.  S.  navy  are  fitted  with  20  or  30  kilowatts  arc  transmitters 
operating  at  the  wave  length  of  4,000  meters.  These  sets  can  intercommunicate  over  dis- 
tances of  2,000  miles  in  daylight. 

216.  The  Alexanderson  High  Frequency  Alternator. — A  2  K.  W.  alter- 
nator (Fig.  280-a)  has  been  developed  by  the  General  Electric  Company  which 
generates  current  at  a  frequency  of  100,000  cycles  per  second,  the  rotor  of  the 
alternator  being  driven  at  a  speed  of  20,000  R.  P.  M.  In  this  particular  type  of 
generator  both  the  armature  and  field  windings  are  stationary,  the  rotor  consist- 
ing merely  of  a  steel  toothed  disc.  Current  at  15  amperes  and  130  volts  is 
obtained  from  the  2  K.  W.  machine,  which  may  flow  directly  into  the  antenna 
circuit  or  may  be  induced  therein  by  an  oscillation  transformer.  In  the  inductive 
method,  shown  in  Fig.  280,  currents  of  radio-frequency  flow  through  the  primary 
winding  of  the  oscillation  transformer  P,  the  secondary  S  being  connected  in 
series  with  the  antenna.  Signalling  is  accomplished  by  placing  a  telegraph  key 
in  series  with  the  field  winding  of  the  alternator  or  by  variation  of  the  localized 
inductance  in  the  antenna  circuit,  as  with  the  arc  system. 

In  the  diagram,  Fig.  280,  the  excitation 
field  winding,  F,  consists  of  a  single  coil, 
wound  inside  the  frame  of  the  entire  ma- 
chine. The  rotor  is  a  toothed  disc  with 
300  projections,  the  space  between  teeth 
being  filled  with  a  non-magnetic  material 
to  cut  down  wind  friction. 

The  armature  coils,  R,  R,  consist  each  of 
a  single  turn  of  wire  wound  zigzag  in 
slots,  and  any  consecutive  pair  may  be  said 
to  constitute  a  pair  of  coils  of  one  turn 
each. 

Current  is  induced  in  the  armature  coils 
as  follows:  When  a  tooth  on  the  rotor 
comes  opposite  the  armature  cores,  A,  B, 
the  magnetic  flux  passes  through  windings 
R,  R,  but  a  minimum  of  flux  threads 
through  the  coils,  when  the  tooth  lies  be- 
tween two  armature  coils.  Thus  an  oscil- 
latory current  is  induced  in  R,  R,  the  fre- 
quency of  which  is  determined  by  the  speed 


G   E.    CONTROL 
COILS 


Fig.     281 — Showing    the     Fundamental     Circuit    of    a 

Radio-Frequency  Control   Device  Developed  by 

the  General  Electric  Company. 

of  the  rotor  and  the  number  of  field  poles. 

A  unique  method  for  controlling  the  antenna  current  of  an  undamped  transmitter  has 
been  developed  by  the  engineers  of  the  General  Electric  Company,  the  fundamental  prin- 
ciple of  which  is  shown  in  Fig.  281.  In  this  system,  a  specially  constructed,  magnetically 
saturated,  amplifying  coil  is  connected  in  shunt  to  the  radio-frequency  alternator,  and  if  a 
telegraph  key  is  inserted  in  the  direct  current  circuit  as  at  K  and  proper  adjustment  made 
of  the  circuit  connected  to  battery  B,  the  effect  of  opening  and  closing  the  key  is  as 
follows : 

When  the  telegraph  key,  K,  is  up  (the  contacts  at  the  rear  of  the  lever  closed),  the 
iron  core,  by  proper  selection  of  ampere  turns  of  the  coil  M-2,  becomes  magnetically  satu- 
rated, and  the  self-inductance  of  the  winding  M-l  is  practically  that  of  a  'simple  coil  without 
an  iron  core.  When  the  telegraph  key  is  pressed  and  the  control  circuit  of  battery  B  opened, 
the  iron  core,  in  respect  to  the  winding  M-l  becomes  magnetic,  and  its  self-induction  is  at  a 
maximum.  In  this  position  of  the  telegraph  key,  the  voltage  across  the  terminals  of  the 
radio-frequency  alternator  is  maximum  and  maximum  current  flows  in  the  antenna  circuit. 
In  this  way  the  dots  and  dashes  of  the  Morse  telegraph  code  can  be  readily  formed,  and 
in  fact,  the  signals  may  be  transmitted  at  an  extremely  high  speed  with  a  practically  non- 
arcing  control. 

Practically  no  current  is  induced  in  the  winding  M-2  (the  control  winding)  because 
it  includes  both  legs  of  the  core  of  winding  M-l.  It  is  necessary  in  this  system  that  the 
ampere  turns  of  windings  M-l  and  M-2  be  practically  equal. 

'Alternate  arrangements  of  this  circuit  are  in  use,  which  are  fully  described  in  the  April,  1916, 
Proceedings  of  the  Institute  of  Radio  Engineers. 


TRANSMITTERS  OF  UNDAMPED  OSCILLATIONS. 


269 


270 


PRACTICAL  WIRELESS  TELEGRAPHY. 


For  the  transmission  of  speech,  a  microphone  transmitter  may  be  connected  in  the  circuit 

of  B  and  wireless  telephony  carried  on  at  large  powers. 

A  photograph  of  a  75  K.  W.  high  frequency  alternator  of  the  Alexanderson  type  appears 

in  Fig.  282.     So  far,  alternators  of  this  capacity  have  been  used  for  laboratory  experiments 

only,  but  there  is  no  reason  to  believe  that  they  will  not  prove  a  commercial  success,  at  least 

at  high  power  land  stations. 

Radio-frequency  alternators   for  the  production  of  lower  frequencies,  in  the  region  of 

30,000  cycles  per  second,  are  less  difficult  to  construct  than  the  type  just  described,  on  ac- 
count of  the  reduced 
peripheral  speed  of  the 
armature.  An  alterna- 
tor, for  example,  gen- 
erating current  at  a  fre- 
quency of  30,000  cycles 
per  second  would  excite 
an  aerial  having  a  natur- 
Lj  al  wave  length  of  10,000 
meters  and  provided  the 
proper  output  could  be 
obtained  would  be  of 
great  use  for  land  station 
work,  but  an  alternator 
having  a  normal  speed  of 
20,000  revolutions  per 
minute  obviously  is  not 
practical  for  continuous 
service  at  either  ship  or 
shore  stations  where  a 
twenty-four-hour  oper- 
ating schedule  is  in  ef- 
fect. It  is  just,  however, 
to  state  that  high  speed 
alternators  such  as  the 
Alexanderson  type  are 
monuments  of  engineer- 
ing skill,  and  many  me- 
chanical and  electrical 


vvvvW 


—  problems,   heretofore 

R  considered  impossible  of 

Fig.    283 — Complete    Circuit    of    the    Goldschmidt    Alternator.  solution,  have  been  com- 

pletely solved. 

217.  Goldschmidt  Radio-frequency  Alternator. — By  adopting  the  simple 
principles  of  electrical  resonance,  Dr.  Goldschmidt  has  designed  an  alternator 
which,  by  the  use  of  a  single  armature,  generates  radio  current  at  frequencies 
from  30,000  to  75,000  cycles  per  second.  The  great  advantage  of  this  alternator 
is  the  low  speed  of  the  armature,  which,  even  in  the  200  K.  W.  sizes,  does  not 
exceed  3,100  R.  P.  M. 

It  will  be  observed  from  diagram,  Fig.  280,  that  in  addition  to  the  stator  and 
rotor  windings  of  the  usual  alternator,  a  number  of  external  inductance  coils 
and  condensers  are  connected  in  both  the  shunt  and  rotor  circuits,  which  are 
employed  for  tuning  these  circuits  to  several  frequencies.  In  fact,  by  correctly 
proportioning  the.  inductance  and  capacity,  current  at  a  frequency  of  60,000 
cycles  per  second  can  be  taken  from  the  field  winding,  and  if  made  to  flow  in  a 
properly  tuned  antenna  circuit  will  radiate  waves,  5,000  meters  in  length. 

In  the  diagram,  Fig.  283,  the  field  windings  of  the  alternator  are  represented  at*  F  (gen- 
erally referred  to  as  the  stator)  and  the  armature  winding  at  A  known  as  the  rotor.  The 
armature  inductance  A  and  the  condenser  C-4  are  shunted  by  the  inductance  L-3  and  the 
condenser  C-5.  Similarly,  L-3  and  C-4  are  shunted  by  condenser  C-3.  The  values  of  C-4, 
L-3,  C-5  and  A  are  so  chosen  that  either  C-4  and  A,  or  C-5  and  L-3  are  in  resonance  to  the 
initial  frequency  of  the  generator,  that  is  to  say,  C-4  and  A  or  C-5  and  L-3  have  identical 


TRANSMITTERS  OF  UNDAMPED  OSCILLATIONS.  271 

time  periods  and  since  they  are  connected  in  parallel,  the  period  of  the  circuit  remains 
unchanged  and  in  resonance  with  the  initial  frequency  of  the  alternator.  The  function  of 
inductance  L-3  and  condenser  C-4  is  to  place  the  armature  on  short-circuit  at  a  given 
frequency.  In  fact,  it  would  not  be  possible  to  completely  short-circuit  the  rotor  winding 
for  radio-frequent  currents  by  merely  placing  a  short  piece  of  wire  across  the  terminals. 
However,  by  placing  another  circuit  containing  inductance  and  capacity  of  identical  time 
period  in  shunt  to  the  rotor,  a  complete  short-circuit  is  effected. 

The  circuit  of  the  stator  F,  C-l,  L-2  and  C-2,  are  similar  to  those  of  the  rotor  A,  but  this 
circuit  is  tuned  to  twice  the  fundamental  frequency  of  the  alternator.  Also,  the  distributed 
capacity  of  the  antenna  system  takes  the  place  of  the  condenser,  C-3,  of  the  rotor  circuit. 

The  field  winding  of  the  stator,  F,  primarily  magnetized  by  the  source  of  direct  current, 
B,  has  the  choking  coil,  L-l,  connected  in  series  to  prevent  the  oscillations  of  radio-frequency 
flowing  back  to  the  exciter  B.  A  regulating  rheostat  R  permits  close  variation  of  the  field 
current. 

It  will  assist  the  student  to  gain  an  understanding  of  the  operation  of  this  generator,  if 
he  will  but  consider  the  following  well-known  phenomenon :  If  a  common  alternating  cur- 
rent generator  armature  revolves  at  such  speed  as  to  generate  an  initial  current  at  a  fre- 
quency of  60  cycles  per  second,  and  the  field  coils,  for  example,  are  mechanically  rotated  in 
the  opposite  direction  at  a  similar  speed,  it  is  plain  that  the  relative  motion  is  doubled  and 
therefore  the  frequency  will  be  doubled.  It  is  also  established  that  we  might  dispense  with 
the  revolving  appliances  for  the  field  coils,  and  pass  an  alternating  current  of  a  given 
frequency  through  a  stationary  field  winding. 

If  the  speed  of  the  armature  is  N1  revolutions  per  second  and  the  frequency  of  the  field 
current  N  cycles  per  second,  there  will  be  induced  in  the  armature  E.  M.  F.'s  of  two  fre- 
quencies, one  of  N  -f-  N1,  and  another  of  N  —  N1. 

In  case  N  and  N1  are  just  equal,  then,  the  frequency  of  the  current  induced  in  the  arma- 
ture will  be  N  -j-  N1  or  2N,  and  N  —  N1  or  zero.  Hence,  if  current  of  60  cycles  flows 
through  a  four  pole  field  winding  and  the  armature  is  driven  at  30  revolutions  per  second, 
current  at  a  frequency  of  120  cycles  per  second  will  be  generated  in  the  armature. 

The  student,  having  some  knowledge  of  the  fundamental  principles  of  the  induction 
motor,  is  well  familiar  with  the  fact  that  a  rotating  magnetic  field  is  produced  by  an  alter- 
nating current  flowing  through  a  stationary  field  winding.  Such  a  current  flowing  through 
the  stator  of  a  generator,  for  instance,  will  give  rise  to  two  oppositely  rotating  magnetic 
fields  of  angular  velocity  corresponding  to  the  frequency  of  the  current,  and  if  the  speed 
of  the  armature  is  carefully  adjusted  to  the  velocity  of  the  field,  it  will  cut  through  one  of 
these  fields  and  thereby  double  the  frequency,  but  it  will  be  stationary  in  respect  to  the  other 
field. 

By  employing  several  generators  to  step-up  the  frequency,  this  principle  might  be  ex- 
tended further.  For  instance,  current  of  a  certain  frequency  generated  in  the  armature  of 
generator  No.  1  could  be  passed  through  the  field  winding  of  generator  No.  2  and  by  proper 
selection  of  armature  speeds,  the  frequency  would  be  doubled  in  the  second  generator, 
and  so  on.  But,  owing  to  the  number  of  steps  involved  in  such  frequency  transformation, 
there  would  be  serious  current  losses  which  could  not  be  compensated  for.  However,  Gold- 
schmidt's  alternator  accomplishes  this  in  a  single  armature  and  by  adoption  of  the  principles 
of  electrical  resonance,  several  frequencies  are  generated  in  the  armature  and  field  windings. 
The  current  of  one  of  these  frequencies  is  selected  and  diverted  to  the  antenna  system  where 
part  of  the  energy  is  radiated  in  the  form  of  electromagnetic  waves. 

The  design  of  Goldschmidt's  alternator  is  such  that  when  driven  at  normal  speed,  the 
initial  frequency  of  the  current  generated  in  rotor  A  is  15,000  cycles  per  second.  Since  the 
inductance  and  capacity  of  the  rotor  circuit  are  selected  to  give  it  a  natural  time  period  of 
oscillation  suitable  to  the  fundamental  frequency,  an  alternating  current  of  considerable 
intensity  flows  (in  the  armature  windings  A,  C-4,  L-3,  C-5). 

The  field  of  the  rotor  corresponding  to  this  frequency  is  composed  of  two  component 
magnetic  fields  of  equal  intensity,  which  rotate  in  opposite  directions  in  respect  to  the  wind- 
ings of  the  rotor.  The  velocity  of  one  of  these  fields  is  zero  in  respect  to  the  rotor,  but  the 
other  travels  at  twice  synchronous  speed  and  therefore  induces  in  the  circuits  of  the  stator 
an  E.  M.  F.  at  a  frequency  of  30,000  cycles  per  second  (and  zero  frequency)  (N  -f  N  and 
N  —  N  or  2N  and  0).  And  since  the  circuit  F,  C-l,  L-2  and  C-2  is  tuned  to  this  frequency, 
current  of  considerable  amplitude  flows. 

Now  the  E.  M.  F.  of  30,000  cycles,  induced  in  the  stator,  induces  in  the  rotor  two  fre- 
quencies, one  of  45,000  cycles  per  second  and  another  of  15,000  cycles  (2n  -f  N  and  2N  —  N 


272 


PRACTICAL  WIRELESS  TELEGRAPHY. 


or  3N  and  N).  The  current  of  45,000  cycles  flows  through  the  condenser  C-3,  but  the 
current  of  15,000  cycles  can,  by  proper  design  of  the  circuits,  be  made  to  nearly  neutralize 
the  former  current  of  15,000  cycles  in  the  rotor.  There  remains,  therefore,  in  the  rotor,  an 
E.  M.  F.  at  a  frequency  of  45,000  cycles  per  second  which  flows  through  the  condenser  C-3, 
and  due  to  a  well-defined  resonant  adjustment,  a  powerful  current  flows. 


DIRECTION  OF 
CURRENT  i  CYCLED 


n=  10,000 


Fig.   284—  The  Joly-Arco  Undamped  Wave   Transmitter. 


The  current  of  45,000  cycles  flowing  in  the  rotor,  will  induce  in  the  winding  of  the  stator, 
current  at  30,000  cycles  per  second  and  60,000  cycles  (3N  —  N  and  3N  -f-  N  or  2N  and  4N). 
This  -current  of  30,000  cycles  nearly  neutralizes  the  former  current  of  30,000  cycles,  leaving 
in  the  circuits  of  the  stator,  current  at  a  frequency  of  60,000  cycles,  to  which  the  antenna  is 
carefully  tuned.  Because  the  impedance  of  L-2,  C-2,  is  considerably  greater  than  that  of 
the  antenna,  the  current  of  4N  oscillates  in  the  antenna  circuit. 

Signalling  is  accomplished  in  the  Goldschmidt  system  by  placing  a  key  in  the  exciter  cir- 
cuits, the  circuit  being  alternately  opened  and  closed. 

An  alternator  of  this  type  installed  at  the  radio  station,  Tuckerton,  New  Jersey,  U.  S.  A., 
is  employed  for  transatlantic  communication  with  a  corresponding  station  located  at  Han- 


TRANSMITTERS  OF  UNDAMPED  OSCILLATIONS.  273 

over,  Germany.  Although  capable  of  generating  200  K.  W..  it  is'normally  used  at  100  to  150 
K.  W.  The  driving  motor  is  of  250  H.  P.  capacity,  operated  from  220  volts  direct  current 
and  has  a  speed  of  4,000  R.  P.  M. 

The  armature  of  the  generator  is  constructed  of  very  thin  laminated  iron  and  revolves 
within  1-32  of  an  inch  from  the  field  poles.  Oil  is  pumped  to  the  bearings  at  considerable 
pressure  and  afterwards  cooled  through  a  refrigerating  machine. 

Operated  at  the  wave  length  of  7,400  meters  with  a  generator  input  of  175  K.  W.,  135 
amperes  flow  into  the  antenna  at  the  Tuckerton  station.  When  atmospheric  electricity  is 
not  severe,  communication  is  maintained  with  Hanover,  Germany,  but  the  stations  operate  at 
a  disadvantage  because  they  are  not  duplexed  as  are  the  Marconi  Transoceanic  stations. 

218.  The  Joly  System  for  the  Production  of  Undamped  Oscillations. — The 

U.  S.  Radio  station  at  Sayville,  Long  Island,  employs  the  Joly-Arco  system  for  the  produc- 
tion of  continuous  waves.  In  this  system  a  specially  designed  generator  having  an  initial 
frequency  of  10,000  or  15,000  cycles  per  second  is  connected  to  two  transformers  having 
magnetically  saturated  iron  cores  which  are  designed  to  double  or  triple  the  frequency  of 
the  alternator. 

A  fundamental  circuit  diagram  of  this  system  is  shown  in  Fig.  284,  wherein  an  alternator 
N  having  an  initial  frequency  of  15,000  cycles  per  second  is  connected  to  the  two  specially 
designed  transformers,  T  and  T-l.  Transformer  T  has  the  excitation  winding  S,  which 
saturates  the  core  fully,  a  similar  winding  S-l  being  provided  for  the  transformer  T-l. 
A  source  of  direct  current  for  the  excitation  windings  S  and  S-l  is  indicated  at  B  with 
the  regulating  rheostat,  R,  connected  in  series.  The  current  generated  by  the  alternator  N 
flows  through  the  primary  windings  C  and  A,  the  complete  primary  circuit  being  tuned  to 
resonance  with  the  initial  frequency  of  the  alternator  by  the  variable  inductance  and  the 
variable  condenser  connected  in  series  with  the  circuit. 

The  secondary  circuit,  or  antenna  system,  embraces  the  windings  D  and  B  connected  in 
series  with  the  antenna  inductance  L,  the  earth  connection  E  and  the  aerial  wires  W.  The 
hot  wire  ammeter  M  is  connected  in  series  with  the  earth  lead  to  determine  conditions  of 
resonance.  As  intimated  previously,  the  transformers  T  and  T-l  are  fully  saturated,  the 
magnetization  being  brought  to  the  knee  or  bend  of  the  characteristic  saturation  curve  by 
carefully  regulating  the  rheostat  R. 

To  illustrate  the  functioning  of  this  apparatus,  let  us  assume  that  the  primary  coils  are 
wound  on  the  cores  so  that  the  direction  of  the  magnetic  lines  of  force  will  be  that  indicated 
by  the  full-line  arrows;  furthermore,  assume  that  at  a  particular  half  cycle  of  current  from 
the  alternator,  the  direction  of  the  current  through  the  primary  winding  is  such  that  the 
corresponding  magnetic  flux  flows  in  the  direction  indicated  by  the  broken-line  arrows ;  then 
the  normal  flux  of  the  core  T  will  not  be  increased  because  this  core  is  already  saturated 
fully,  the  added  flux  flowing  in  the  direction  of  the  core  flux,  but  the  normal  flux  of  the 
core  T-l  will  be  opposed  by  the  flux  of  the  winding  A  and  the  total  lines  of  force  flowing 
through  the  core,  therefore,  reduced.  This  reduction  of  flux,  followed  by  subsequent  rise 
to  normal  saturation  upon  the  completion  cf  an  alternation  from  N,  causes  two  changes  of 
flux  through  coil  B  of  transformer  T-l,  resulting  in  the  production  of  two  alternations  in 
that  winding  for  one  alternation  of  current  from  N.  The  final  effect  is  to  induce  current  of 
double  frequency  in  the  antenna  system. 

Let  the  next  half  cycle  of  the  alternator  N  be  completed  through  windings  A  and  C ; 
then,  the  change  of  flux  takes  place  in  the  core  of  the  transformer  T,  rather  than  in  T-l, 
resulting  in  the  induction  of  two  alternations  of  current  in  the  secondary  winding  D.  Sum- 
ming up  the  foregoing,  a  complete  cycle  of  current  from  the  generator  N  will  induce  two 
alternations  of  current  in  the  coil  B,  followed  by  two  alternations  of  current  in  the  coil  D, 
thereby  doubling  the  initial  frequency  of  the  generator  current.  This  current  flows  in  the 
antenna  circuit  which  has  been  carefully  tuned  to  resonance,  and  part  of  the  energy  is  con- 
verted into  electric  waves. 

By  an  additional  set  of  transformers,  the  current  of  double  frequency  generated  by  the 
first  set  of  the  transformers  may  be  again  doubled,  but  the  efficiency  of  the  apparatus,  as  a 
whole,  decreases  considerably  as  the  steps  of  transformation  are  increased. 

Another  system  of  transformation  has  been  evolved  by  Joly,  in  which  the  frequency  ot 
the  generator  may  be  tripled  by  a  single  set  of  transformers,  which  are  adjusted  to  have — 
when  A.  C.  current  flows — unequal  degrees  of  saturation.  In  this  system  the  D.  C.  excita- 
tion winding  for  the  transformer  is  dispensed  with. 

In  systems  of  this  type  in  order  that  resonance  between  the  alternator  current 
and  the  antenna  system  may  be  maintained,  it  becomes  important  that  the  fre- 


274 


PRACTICAL   WIRELESS   TELEGRAPHY. 


quency  of  the  alternator  may  remain  constant ;  in  consequence,  the  speed  of  the 
driving  motor  must  be  carefully  governed  and  to  this  end  several  devices  have 
been  brought  forth,  the  principal  one  being  a  signalling  key  fitted  with  a  special 
set  of  contacts  which,  just  previous  to  the  closing  of  the  key,  add  resistance  in 
the  motor  field  circuit,  maintaining  a  practically  uniform  speed  of  rotation. 


Fig.    285 — Marconi's   Method   of   Generating   Continuous   Oscillations. 


Signalling  is  accomplished,  in  the  Joly  system,  by  inserting  a  key  in  series  with  the  direct 
current  excitation  winding,  or  by  interrupting  the  generator  circuit  to  the  primary  winding 
of  the  transformers  by  a  special  electromagnetic  key,  or  by  cutting  in  and  out  a  few  turns 
of  the  aerial  tuning  inductance.  The  first-named  method  is  the  one  in  use  at  present.  The 
alternator  at  the  Sayville  station  has  a  capacity  of  100  kilowatts  and  gives  antenna  current 
of  about  140  amperes.  Fair  communication  is  effected  with  the  station  at  Nauen,  Germany, 
throughout  the  24  hours  of  the  day,  but  the  best  results  have  been  obtained  during  the  dark 
hours.  At  present  the  Sayville  station  operates  at  the  wave  length  of  9,400  meters,  the 
antenna  frequency  being  slightly  above  30,000  cycles  per  second. 

219.  Marconi's  System  for  the  Production  of  Continuous  Waves. — Signer 
Marconi  and  his  engineering  staff  have  developed  an  ex- 
ceedingly novel  method  for  generating  continuous  oscilla- 
tions, doing  away  with  many  of  the  problems  encountered 
in  the  construction  of  intricate  High  frequency  alternators. 
The  fundamental  principle  upon  which  the  system  is  based  is 
disclosed  in  the  diagram,  Fig.  285,  wherein  a  number  of  disc  dis- 
chargers, D-l,  D-2,  D-3,  D-4,  are  mounted  on  a  common  shaft, 
each  of  which  is  connected  to  an  oscillation  circuit,  such  as  P-l 
and  C-l,  P-2  and  C-2,  etc.  The  main  condensers,  C-l,  C-2,  C-3, 
C-4,  are  charged  by  a  source  of  high  potential  direct  current,  indi- 
cated at  G,  consisting  of  two  5,000  volt  generators  connected  in 
series.  The  disc  dischargers,  D-l,  D-2,  etc.,  are  set  on  the  shaft  so 
that  the  condensers  discharge  and  recharge  at  regular  intervals  in 
succession,  and  if  the  velocity  of  the  discs  is  adjusted  so  that  the 
interval  between  the  beginning  of  the  discharge  of  one  condenser 
and  the  beginning  of  the  discharge  of  the  next  condenser  is  equal 
to  the  period  of  the  oscillations  in  the  antenna  circuit,  or  an  exact 
multiple  thereof,  the  oscillations  flowing  in  the  antenna  circuit  will 
overlap,  producing  continuous  oscillations  of  practically  constant 
amplitude,  as  shown  in  Fig.  286. 

To  insure  that  each  condenser  discharges  at  the  right  moment, 
and  that  the  successive  groups  of  oscillations  induced  in  the  antenna 
circuit  are  in  phase,  the  discharge  circuit  is  provided  with  an  auxiliary  timing  rotary  disc 
discharger,  known  as  the  trigger  or  timing  disc  (not  shown  in  the  drawing),  which  performs 
the  function  of  setting  off  the  main  spark  discharge  at  the  proper  time. 


Fig.  286— Showing  How 
Groups  of  Damped  Oscil- 
lations Can  Be  Made  to 
Overlap  One  Another. 


TRANSMITTERS  OF  UNDAMPED   OSCILLATIONS. 


275 


A  modified  form  of  the  timed-spark  oscillation  generator  is  shown  in  U.  S.  Patent  1,136,477 
of  1915,  one  arrangement  being  that  shown  in  Fig.  287.  Here  a  direct  current  generator  is 
indicated  at  G  and  a  choking  coil  at  L-l.  Since  G  has  an  electromotive  force  of  about  10,000 
volts,  condensers  K-l  and  K-2  receive  a  charge  which  enables  them  to  discharge  across  the 
spark  gap.  A  terminal  of  either  condenser  is  connected  to  the  disc  dischargers,  D-l  and  D-2, 
and  the  studs  of  the  latter  are  placed  so  that  condensers  C-l,  C-2  and  C-3  are  alternately 
and  successively  charged,  let  us  say,  through  inductance  L-2  and  successively  discharged 
and  recharged  through  L-3. 

The  oscillations  set  up  in  L-2  or  L-3  are  made  to  act  inductively  on  an  intermediate 
circuit,  L-4,  L-5  and  C-4,  and  if  the  discs  be  revolved  at  a  certain  speed  and  are  properly 
timed,  the  trains  of  oscillations  induced  in  the  intermediate  circut  overlap  one  another,  re- 
sulting in  a  continuous  flow  of  oscillations  in  the  antenna  circuit,  A-l,  E. 


Fig.  287 — One  Type  of  Marconi   Continuous  Wave   Generator. 

In  ths  system,  signalling  is  accomplished  by  inserting  a  high  tension  relay  key  in  series 
with  the  source  of  direct  current  from  the  generator  or  by  closing  the  circuit  of  the  timing 
disc. 

The  apparatus  for  this  system  has  been  developed  further  by  the  Marconi's  Wireless 
Telegraph  Company,  Ltd.,  and  is  not  used  at  present  as  shown  in  either  Figs.  285,  286  or  287. 
The  details  of  the  more  modern  apparatus  cannot  be  published  for  the  present,  but  it  may 
be  remarked  that  a  transmitting  set  of  this  type  is  in  daily  use  at  the  great  Marconi  station 
at  Carnarvon,  Wales,  England,  and  a  similar  set  will  be  installed  in  the  transoceanic  station 
at  Marion,  Mass.,  U.  S.  A. 

With  a  power  input  of  less  than  100  K.  W.,  the  author  has  copied  the  signals  from  the 
Carnarvon  station  in  New  York  City  during  the  daylight  hours  with  an  aerial  100  feet  in 
length.  The  signals  were  considerably  stronger  than  those  obtained  from  other  foreign 


276 


PRACTICAL   WIRELESS   TELEGRAPHY. 


stations  using  radio-frequency  alternators  of  greater  power.  The  wave  length  employed 
during  these  tests  was  close  to  10,000  meters. 

220.  The  Pliotron  Oscillator. — Highly  exhausted  two  or  three  element 
vacuum  valves  of  enlarged  dimensions  can  be  employed  to  generate  current  at  audio-  or  radio- 
frequencies.  In  the  form  designed  by  the  engineers  of  the  General  Electric  Company,  the 
device  has  been  termed  a  "Pliotron."  The  Pliotron  bulb  contains  a  tungsten  filament  F  (hot 
cathode)  brought  to  a  state  of  incandescence  by  110  or  220  volts  direct  current,  a  grid  of 
tungsten,  G,  and  a  tungsten  plate,  P,  (anode),  as  indicated  in  Fig.  288. 

The  filament  F  is  brought  to  incandescence  by  a  source  of  110  volts  direct  current,  A,  B, 
which  source  also  supplies  current  for  the  anode  circuit  between  the  filament  and  the  plate. 
The  radio-frequency  inductance  of  the  anode  circuit,  indicated  at  L,  is  shunted  by  the 
condenser  C-l.  L-2,  the  radio-frequency  inductance  of  the  grid  circuit,  also  has  the  shunt 
condenser  C-2.  By  variation  of  the  capacity  of  C-l  and  C-2,  the  grid  circuit  and  anode 
circuit  may  be  tuned  to  resonance  and  the  oscillations  flowing  in  the  anode  circuit  repeated 
back  to  and  reinforced  through  the  grid  circuit  by  inductively  coupling  coil  L-l  to  L-2. 
By  means  of  a  third  winding,  L-3,  coupled  to  either  L-l  or  L-2,  current  of  any  desired 

frequency  can  be  gen- 
erated by  proper  varia- 
tion of  inductance  and 
capacity ;  in  fact,  cur- 
rent at  frequencies  from 
60  to  1,000,000  cycles 
per  second  can  be  ob- 
tained. 

The  radio-frequent 
currents  generated  in 
the  circuits  of  the 
Pliotron  can  be  trans- 
ferred to  the  antenna 
system  by  connecting 
the  terminals  of  the 
coil  L-3  to  the  antenna 
and  earth  wire,  the 
precaution  being  taken 
to  tune  the  aerial  cir- 
cuit to  resonance  with 
the  local  current.  A 
number  of  Pliotrons 
may  be  connected  in 
parallel  and  direct  cur- 
rent at  any  desired 
power  converted  into 

^^^  oscillations     of     radio- 

^ST  frequency. 

The  Pliotron  vacuum 

Fig.    288 — The    Pliotron    Oscillator    Connected    for    the    Production    of    Radio-    bulb  Can  also  be  Used  to 
Frequency  Oscillations.  yary    ^    ampljtude    of 

the  oscillations  in  an  antenna  system  traversed  by  undamped  oscillations.  If  properly  con- 
nected, it  is  applicable  for  radio-telephony  as  well  as  telegraphy.  If  the  anode  of  the  Pliotron 
is  connected  to  a  point  of  high  potential  in  the  antenna  system  and  the  negative  side  of 
the  filament  to  earth,  and  the  grid  potential  of  the  Pliotron  is  strongly  negative,  no  leakage 
to  earth  takes  place,  but  if  the  negative  potential  is  decreased,  sufficient  energy  will  be  with- 
drawn from  the  antenna  to  strongly  damp  the  oscillations.  In  fact,  practically  all  the  energy 
of  the  antenna  oscillations  can  thus  be  diverted  to  earth. 

If  an  electromotive  force  of  the  correct  value  is  impressed  upon  the  grid  through  a 
small  telegraph  key,  the  dots  and  dashes  of  the  Morse  code  are  formed  by  simply  closing 
the  circuit  and  thus  varying  the  grid  potential. 


PART  XV. 

RECEIVERS  FOR  UNDAMPED  OSCILLA 
TIONS  OR  CONTINUOUS  WAVES. 

221.  THE  PROBLEM.  222.  THE  TIKKER.  223.  THE  HETERO- 
DYNE SYSTEM.  224.  THE  VACUUM  VALVE  AS  A  SOURCE  OF 
RADIO-FREQUENCY  OSCILLATIONS.  225.  VACUUM  VALVE  AS  A 
COMBINED  DETECTOR,  AMPLIFIER  AND  BEAT  RECEIVER.  226. 
OSCILLATING  VACUUM  VALVE  DETECTOR  CIRCUITS  OF  THE  U.  S. 
NAVY.  227.  THE  GOLDSCHMIDT  TONE  WHEEL.  228.  MARCONI 
SYSTEM  FOR  RECEPTION  OF  UNDAMPED  OSCILLATIONS. 


(4) 


221.  The  Problem. — If  an  ordinary  crystal  rectifier  be  connected  to  a 
receiving  set  tuned  to  a  continuous  wave  transmitter,  owing  to  the  lack  of  discon- 
tinuity in  the  advancing  wave,  a  continuous  pulsating  current  flows  through  the 
receiving  telephone.  These  pulsations  take  place  at  such  rapid  rates  that  the 
diaphragm  of  the  telephone  is  either  held  down  continuously  or  repelled  continu- 
ously resulting  in  no  sound  except  at  the  beginning  or  end  of  the  flow. 

To  make  undamped  oscillations  audible,  we  are  compelled  to  break  up  the 
oscillations  of  either  the  transmitter  or  receiver  into  groups  suitable  for  maximum 
response  in  the  head  telephone  or  to  supply  other  means  at  the  receiver  to  make 
them  audible. 

The  receivers  at  present  in  use  are : 

(1)  The  Poulsen  tikker  or  chopper; 

(2)  The  "Heterodyne"  system; 

(3)  The  Goldschmidt  Tone  Wheel; 

The  Regenerative  Vacuum  Valve  (used  as  a  "beat"  receiver). 

The  mode  of  func- 
tioning of  these  detectors 
is  briefly  described  as 
follows :  The  Poulsen 
tikker  interrupts  the  cir- 
cuits of  the  receiving 
tuner  at  a  uniform  rate 
per  second  (approxi- 
mately 300  to  600  times}  ; 
the  heterodyne  system  is 
based  upon  the  interac- 
tion of  two  radio-fre- 
quent currents  in  the 
receiving  aerial,  resulting 
in  the  production  of  an 
audio-frequent  current  in 
the  telephone  circuit;  the 
Goldschmidt  tone  wheel 
converts  the  current  of 
radio-frequency  (the  in- 

PHONES  coming  oscillations)  into 

an  audio-frequent  current 
and    the    regenerative 

Fig.   289— The    Simple   Poulsen    Tikker   and  Circuit.  "beat"    receiver    employs 


278 


PRACTICAL  WIRELESS  TELEGRAPHY. 


the  vacuum  valve  as  an  oscillator  at  radio-frequencies  to  produce  the  "heterodyne"  effect. 
This,  combined  with  the  well-known  relaying  action  of  the  vacuum  valve  and  its  ability  to 
repeat  currents  of  radio-frequency,  makes  a  receiver  of  unusual  sensitiveness. 

222.  The  Tikker. — In  its  commercial  form  the  tikker    consists  of  a  com- 
mutator interrupter,  the  principal  of  which  is  shown  in  Fig.  289. 

A  disc  D  mounted  on  the  shaft  of  the  motor  M  has  a  number  of  teeth  filled  in  between 
with  insulating  material  such  as  fibre.  The  radio-frequent  currents  flow  from  brush  B  to  A 
through  the  disc  which  interrupts  them  from  300  to  1,000  times  per  second.  The  charge  built 
up  in  the  condenser,  C-l,  by  resonance  with  the  aerial  system  discharges  into  the  telephone 
condenser,  C-2  (Fig.  289),  at  regular  intervals.  Condenser  C-2,  in  turn,  discharges  through 
the  head  telephone  creating  a  single  sound  for  the  charge  accumulated.  Due  to  the  fact 
that  the  tikker  discharges  C-l  at  various  potentials,  or  we  might  say,  at  different  points  on 
the  cycle  of  the  incoming  oscillations,  a  non-uniform  note  is  produced  lacking  the  desired 
musical  pitch  for  reading  through  atmospheric  electricity.  However,  the  tikker  suffices  as  a 
simple  receiver  and  good  results  have  been  obtained  by  its  use  at  several  ship  and  shore 
stations. 

A  favored  form  of  circuit  interrupter  for  the  reception  of  undamped  oscillations  is  the 
sliding  -ivire  or  slipping  contact  detector,  shown  in  Fig.  290.  In  the  diagram  a  small  brass 
wheel,  W,  mounted  on  the  shaft  of  the  motor,  M,  is  in  contact  with  the  brush  B.  The 
terminals  of  the  slipping  contact,  B,  are  connected  in  the  circuit  of  the  tuner  as  the  tikker, 

D,  of  Fig.  289.  The  cir- 

B         mmm  _  cuit    is    completed 

through  the  brush,  B-l, 
in  contact  with  the  shaft 
S.  When  the  wheel  is 
in  rotation,  there  is  a 
constant  slipping  and 
gripping  of  the  brush  B 
on  the  wheel  which 
makes  a  contact  of  vari- 
able resistance,  causing 
the  charge  accumulated 
in  the  telephone  con- 
denser to  vary  in  ac- 
cordance. The  note  in 
the  telephone  will  have 
a  pitch  in  accordance. 
Both  the  tikker  and 
the  slipping  contact  de- 
tectors are  considered 

as  "current  operated"  devices,  meaning  that  they  function  best  in  a  tuning  circuit  which 
affords  a  maximum  of  current  rather  than  a  high  voltage.  The  coils  of  the  tuner  for  these 
oscillation  detectors  are  wound  with  Litzendraht  wire  or  conductors  of  equal  high  frequency 
conductivity. 

These  detectors  are  applicable  to  the  reception  of  damped  oscillations  or  discontinuous 
waves,  but,  owing  to  the  discontinuity  of  the  wave  trains,  an  irregular  note,  much  similar 
to  the  discharges  produced  by  atmospheric  electricity,  is  obtained. 

223.  The  Heterodyne  System. — The  functioning  of  the  heterodyne*  re- 
ceiver is  based  upon  the  interaction  of  alternating  currents  of  two  frequencies  in 
some  part  of  the  receiver  circuits. 

For  example,  if  an  alternating  current  of  50,000  cycles  per  second  flows  through  a  given 
circuit,  and  there  is  superposed  on  it  another  current  having  frequency  of  49,000  cycles  per 
second,  a  "beat"  current  will  result  having  a  frequency  equal  to  the  numerical  difference 
of  the  two  applied  frequencies  or  1,000  cycles  per  second.  The  same  frequency  would  be 
obtained  by  superposing  a  current  of  51,000  cycles  upon  another  current  of  50,000  cycles;  in 
fact,  in  any  case,  the  frequency  of  the  beat  current  could  be  the  difference  of  the  two 
applied  frequencies. 

By  adoption  of  this  principle  we  are  afforded  ready  means  for  making  audible  undamped 
oscillations  from  a  given  transmitter  at  the  receiver.  One  method  is  shown  in  Fig.  291, 


Fig.    290— The    "Slipping    Contact"    Detector    for    Reception    of   Undamped 

Oscillations. 


*  Abroad,   this  receiver  is  called  the   "local  interference"   receiver. 


JL  D 


RECEIVERS  FOR  UNDAMPED  OSCILLATIONS  PR  WAVES,  i  279 

where  the  receiver  circuits  are  indicated  by  L-l,  L-2,  the  crystal  rectifier  D,  etc.  An  un- 
damped oscillation  generator  consisting  of  the  D.  C.  arc  gap  A,  B,  the  variable  condenser  C 
and  inductance  L  are  shown  at  the  base  of  the  drawing/  By  means  of  this  generator,  a  steady 
stream  of  oscillations  is  supplied  to  the  aerial  system  through  the  coupling  coil  L-3,  the 
frequency  of  which  can  be  altered  either 
by  the  condenser  C  or  by  the  induct- 
ance L. 

Now,  if  the  receiving  tuner,  for  ex- 
ample, is  tuned  to  the  wave  length  of 
6,000  meters  corresponding  to  an  oscilla- 
tion frequency  of  50,000  cycles  per  second 
(which  may  be  the  wave  length  of  a  given 
transmitting  station),  and  the  arc  genera- 
tor adjusted  to  generate  oscillations  at  a 
frequency  of  49,500  cycles  per  second,  a 
beat  current  of  a  frequency  of  500  cycles 
per  second  will  flow  in  the  circuit  of  the 
receiver  where  it  will  be  rectified  by  the 
detector,  D,  and  made  audible  in  the  head 
telephone  P. 

By  carefully  adjusting  the  local  fre- 
quency, the  beat  note  can  be  varied  from 
a  pitch  equivalent  to  that  of  a  200  cycle 
alternating  current  up  to  the  limits  of 
and  beyond  audibility.  The  interaction  of 
these  two  frequencies  in  the  receiving 
system  not  only  makes  undamped  oscilla- 
tions audible  but  also  amplifies  the  re- 
ceived signal  to  a  marked  degree.  Even 
damped  oscillations  may  be  amplified  by 
this  means,  but  in  this  case  the  normal 
note  of  the  spark  transmitter  is  distorted, 


UNDAMPED   OSCILLATION 
GENERATOR 


MO  VOLTS    O.C. 


Fig.  291 — Early  Form  of  Heterodyne  Receiver. 


the  resultant  note  being  of  lower  pitch.    The  note  of  the  local  beat  current  generated  by  the 
heterodyne  receiver  has  the  most  uniform  pitch  when  the  sending  station  employs  a  radio- 
frequency    alternator;    although    flute- 
OSCILLATIONS  ^e  tones  are  obtained  from  arc  trans- 

mitters, the  "beat"  current  does  not 
have  anywhere  near  the  musical  pitch 
that  can  be  obtained  from  the  radio- 
frequency  alternator.  The  arc  gener- 
ator of  Fig.  291  might  be  replaced  by 
a  radio-frequency  alternator  of  small 
output  or  by  a  buzzer  excitation  sys- 
tem, but  a  generator  giving  genuinely 
undamped  oscillations  is  preferred. 

The  precise  actions  taking  place  in 
the  heterodyne  receiver  of  Fig.  291  can 
be  explained  by  the  series  of  curves  in 
Fig.  292.  The  oscillations  O-l,  indi- 
cated on  the  upper  line,  are  those  in- 
coming at  a  given  station  (without  the 
local  arc  generator  in  operation)  while 
those  on  the  second  O-2  correspond  to 
the  frequency  of  the  arc  generator  and 
are  of  a  lower  order  of  frequency  than 
the  incoming  oscillations. 

The  third  curve  O-3  indicates  the 
resultant  "beat"  current  due  to  the  in- 
teraction  of   the   local  frequency   and 
that  of  the  incoming  signal.    This  cur- 
Fig.  292-Curves  Showing  the  Functioning  of  the  Heterodyne    rent     haSr   ZG™     ™1UC     whe"     *«     ^O 

Receiver.  groups  of  oscillations  oppose  and  max- 


280 


PRACTICAL   WIRELESS   TELEGRAPHY. 


RECEIVER    CIRCUIT 


imum  value  when  they  assist  or  are  in  phase,  and  the  frequency  of  the  beat  current  is  the 
numerical  difference  of  the  frequency  of  the  incoming  oscillations  and  that  of  the  local 
generator. 

When  rectified  by  the  crystal,  the  successive  cycles  of  the  beat  current  take  the  form  of 
the  curve  O-4,  where  the  negative  halves  have  been  cut  off  and  the  positive  halves  remain, 
for  operation  of  the  telephone,  as  shown  by  the  heavy  curved  line  O-5. 

224.  The   Vacuum  Valve  as  a  Source  of  Radio-Frequency  Oscillations. 
— If  a  three  element  vacuum  valve  is  connected  up  as  in  Fig.  293  with  the  correct  values  of 
inductance  for  the  coils  L-4,  L-5,  L-6  and  L-7,  and  for  the  condensers,  C-3,  C-4  and  C-5, 
oscillations  of  radio-frequency  flow  through  L-4,  the  periodicity  of  which  may  be  varied 
principally  by  C-3  and  C-5.     Connected  in  this  manner,  the  vacuum  valve  becomes  a  gen- 
erator of  sustained  oscillations  which  may  interact  with  the  incoming  oscillations  for  the 
production  of  "beat"  currents.     For  the  maximum  strength  of  signals,  the  coil  L-4  bears  a 

certain  critical  coupling 
to  L-3,  and  the  coupling 
between  L-5  and  L-6, 

V  '  I    /  also     requires     care- 

\    /  f ul  adjustment  for  steadi- 

^*  ness  and  constancy. 

The  "beat"  current  set 
up  in  the  receiver  wind- 
ings L-2  and  condenser 
C-2  is  rectified  by  the 
crystal  rectifier  D,  and 
made  audible  in  the  head 
telephone  P. 

Increased  strength  of 
signal  is  obtained  by  the 
use  of  a  single  three  ele- 
ment vacuum  valve  or  a 
triple  valve  amplifier  in 
place  of  the  crystal  recti- 
fier D. 

Since  the  reception  of 
very  long  wave  lengths 
is  involved  in  a  system  of 
this  type,  the  inductances 
L-7  and  L-8  are  of  the 
order  of  90,000  micro- 
henries each.  It  has  been 
found  possible  to  design 
a  beat  receiver  tuner  hav- 
ing fixed  values  of  in- 
ductance throughout,  the 
required  changes  in  fre- 
quency for  "heterodyn- 
ing" being  obtained  by 
variable  condensers. 

225.  Vacuum  Valve  as  a  Combined  Detector,  Amplifier  and  Beat  Receiver. 

— Much  of  the  record-breaking,  long  distance  transmission  effected,  of  late,  is  due  to  the 
vacuum  valve  being  employed  as  a  combined  detector  and  "beat"  receiver.  One  circuit  of 
this  type  is  shown  in  Fig.  294,  wherein  it  will  be  observed  that  the  secondary  circuit,  L-4, 
L-3,  C-2  and  L-5,  can  be  placed  in  resonance  with  the  tertiary  circuit,  L-6,  L-7  and  C-4, 
and  when  the  filament  is  brought  to  the  correct  degree  of  incandescence  by  the  rheostat  R, 
the  discharge  of  condenser  C-4  through  L-7  and  L-6  starts  the  flow  of  undamped  oscilla- 
tions which  are  transferred  to  the  secondary  circuit  through  L-5  and  L-6. 

Due  to  the  relaying  action  of  the  valve,  the  oscillations  are  repeated  back  to  the  grid 
(of  the  valve)  and  accordingly  amplified. 

Now  if  the  antenna  circuit,  L-l,  L-8  and  L-2,  is  adjusted  to  the  frequency  of  the  incom- 
ing oscillations,  and  the  secondary,  as  well  as  the  tertiary  circuit;  adjusted  to  a  slightly 


VACUUM    VALVE    GENERATOR 
Fig.   293 — The  Vacuum  Valve  Heterodyne  Receiver. 


RECEIVERS  FOR  UNDAMPED  OSCILLATIONS  OR  WAVES. 


281 


different  frequency  of  oscillation,  a  beat  current  is  set  up  in  the  secondary  circuit,  which  is 
amplified  in  the  local  telephone  circuit  by  the  charge  placed  upon  the  grid. 

As  explained  by  Armstrong,  the  incoming  oscillations  are  transferred  from  the  antenna 
circuit  to  the  secondary  circuit  in  the  usual  manner,  and  due  to  the  repeating  action  of  the 
valve,  they  are  reproduced  in  the  tertiary  circuit,  coupled  back  to  the  secondary  circuit 
through  the  coupling  transformer,  L-5  and  L-6.  and  re-enforced,  all  this  taking  place  while 
the  valve  is  generating  oscillations  locally.  Simultaneously,  beats  are  produced  by  the 
interaction  of  the  local  oscillations  and  the  incoming  oscillations,  the  effect  being  to  alter- 
nately increase  and  decrease  the  amplitude  of  the  oscillations  in  the  complete  system. 


\/ 


'BEAT'  RECEIVER  FOR  DAMPED  AND  UNDAMPED 
OSCILLATIONS. 


L-a 


Fig.   72 


Fig.   294 — Complete    Circuits   of   a   Beat   "Receiver. 

An  increase  in  the  amplitude  of  the  grid  oscillations,  increases  the  negative  charge  in  the 
grid  condenser,  producing  a  decrease  in  the  average  value  of  the  B-2  battery,  and,  therefore, 
a  decrease  in  the  telephone  current,  but  a  decrease  in  the  amplitude  of  the  local  oscillations 
allows  some  of  the  negative  charge  in  the  grid  condenser  to  leak  off  and  thereby  permit  an 
increase  of  the  telephone  current.  The  result  of  all  this  is  the  production  of  an  alternating 
current  of  audio-frequency,  which  is  practically  a  simple  harmonic  current. 

(a)  Operation  and  Adjustment.  For  wave  lengths  between  6,000  and  10,000  meters,  the 
tuning  for  the  secondary  and  tertiary  circuits  is  done  principally  at  the  condensers,  C-2 
and  C-4.  Also  the  coupling  between  L-5  and  L-6  must  be  carefully  adjusted  for  maximum 
response.  Careful  regulation  must  be  made  of  the  voltage  of  the  B-2  battery,  which 
normally  varies  from  50  to  150  volts,  depending  upon  the  type  of  valve  in  use.  The  vari- 
ometer inductance  L-8  is  a  useful  element  of  the  antenna  system  for  variation  of  its  natural 


282  PRACTICAL   WIRELESS   TELEGRAPHY. 

frequency.  It  affords  a  close  adjustment  of  inductance  between  the  taps  of  the  multi-point 
switch,  and  also  permits  variation  of  the  frequency  of  the  beat  current. 

It  facilitates  the  preliminary  adjustment  of  this  receiver  if  access  can  be  obtained  to  an 
accurate  wave-meter.  The  wave-meter,  shunted  by  a  buzzer  excitation  circuit,  is  placed  in 
inductive  relation  to  the  earth  lead  of  the  antenna  system,  and  the  signals  at  any  particular 
wave  length  of  the  wave-meter  are  recorded  on  the  receiving  apparatus.  In  this  manner  a 
precalibration  is  effected,  eliminating  all  guesswork.  It  is  not  difficult,  however,  to  tune  the 
apparatus  to  an  undamped  transmitter  without  a  wave-meter,  if  certain  precautions  are 
observed.  To  begin  with,  the  coil  L-5  is  placed  in  slight  inductive  relation  to  L-6.  L-4  is 
placed  about  half  way  inside  L-2  and  the  coil  L-7,  generally,  is  used  at  its  maximum  value. 
The  condenser,  C-3,  is  used  at  an  exceedingly  small  value  of  capacity  (very  near  to  zero). 
The  condensers,  C-2  and  C-4,  are  varied  simultaneously  in  capacity  until  a  peculiar  "thump- 
ing" sound  is  heard  in  the  head  telephone.  Near  to  this  point  the  secondary  and  local 
telephone  circuit  are  in  oscillation  and  in  resonance. 

It  is  found  that  the  vacuum  valve  having  less  than  the  maximum  amount  of  exhaustion, 
oscillates  far  below  the  characteristic  "blue  glow"  point.  On  the  other  hand,  this  glow  is  not 
evident,  in  the  highly  exhausted  valves,  regardless  of  the  locally  applied  voltage.  The 
correct  adjustments  throughout,  in  this  type,  are  determined  by  experiment. 

We  may  resume  the  complete  process  of  tuning  a  receiving  set  of  this  type  as  follows: 
The  antenna  system  is  tuned  to  resonance  with  the  incoming  wave,  but  the  secondary  and 
tertiary  circuits  are  set  into  oscillation  at  a  slightly  different  frequency.  Due  to  the  inter- 
action of  the  two  currents,  "beat  currents"  within  the  limits  of  audibility  are  produced  and 
by  variation  of  the  local  frequency,  the  pitch  of  the  note  in  the  head  telephone  may  be  varied 
over  a  considerable  range.  In  manipulating  this  apparatus  it  is  shown,  during  the  reception 
of  undamped  oscillations,  that  proper  adjustment  has  been  attained  when,  if  the  variable 
elements  (the  coils  and  condensers)  of  either  of  the  associated  circuits  of  the  complete 
tuner  are  altered  in  value,  the  pitch  of  the  note  changes  accordingly.  During  the  initial 
adjustment,  and  afterward  the  body  of  the  manipulator  must  be  kept  at  a  considerable 
distance  from  the  high  potential  ends  of  the  tuning  coils,  otherwise  the  circuits  will  be 
thrown  out  of  resonance  and  the  signals  will  disappear. 

With  the  apparatus  of  the  foregoing  type,  signals  have  been  received  in  New  York  City 
from  a  certain  transmitting  station  at  Honolulu  at  a  distance  of  approximately  6,500  miles. 

Dimensions  of  a  receiving  set  of  this  type,  adjustable  to  wave  lengths  up  to  10,000  meters, 
which  has  been  tried  and  tested  experimentally,  follow: 

Referring  to  the  diagram,  Fig.  294,  the  high  voltage  battery,  B-2,  consists  of  from  50  to 
100  miniature  flashlight  cells  adjustable  in  steps  of  three  at  a  time  or  preferably  singly.  The 
filament  battery  B-l  generally  does  not  exceed  four  volts,  although  valves  have  been  con- 
structed having  12  volt  filaments.  The  rheostat  R  has  resistance  varying  from  10  to  20 
ohms.  It  is  essential  that  it  be  closely  adjustable. 

The  inductance  coil  of  the  local  telephone  circuit,  L-7,  is  26  inches  in  length  and  Sl/2 
inches  in  diameter,  wound  closely  with  No.  28  S.  S.  C.  wire.  The  coil,  L-6,  is  6  inches  in 
diameter  by  6  inches  in  length,  wound  closely  with  No.  26  S.  S.  C.  wire.  The  coil,  L-5,  is  6 
inches  in  length,  wound  closely  with  No.  26  S.  S.  C.  wire.  The  coil,  L-5,  is  also  6  inches  in 
length  by  5l/2  inches  in  diameter  and  mounted  so  it  can  be  placed  in  variable  inductive 
relation  with  L-6.  It  is  rarely  necessary  to  place  L-5  completely  inside  of  L-6,  but  the 
coupler  should  be  constructed  to  permit  the  coupling  between  the  two  coils  to  be  adjusted 
over  a  fair  range.  The  coil,  L-5,  is  also  wound  with  No.  26  S.  S.  C.  wire. 

The  secondary  winding  of  the  receiving  tuner,  L-4,  is  5  inches  in  diameter  by  14  inches 
in  length,  covered  with  No.  28  S.  S.  C.  wire.  The  secondary  loading  coil,  L-3,  has  similar 
dimensions  to  the  coil  L-7,  each  of  which  may  be  wound  on  a  cardboard  form  or  hard 
rubber  tube. 

The  secondary  condenser,  C-2,  has  maximum  capacity  of  .0005  microfarads  and  preferably 
is  constructed  so  that  the  zero  position  on  the  condenser  scale  actually  represents  zero 
capacity.  The  grid  condenser,  C-3,  has  the  same  capacity  as  the  condenser,  C-2.  Both 
condensers  should  be  fitted  with  handles  at  least  12  inches  in  length,  in  order  that  the 
constants  of  the  circuit  are  not  changed  by  the  body  of  the  manipulator.  The  condenser, 
C-4,  should  have  a  maximum  capacity  of  .002  microfarads. 

The  primary  winding,  L:2,  is  Sl/2  inches  in  diameter  by  12  inches  in  length,  covered  with 
No.  24  S.  S.  C.  wire.  It  is  somewhat  difficult  to  give  complete  dimensions  for  the  aerial 
tuning  inductance,  L-l,  unless  the  inductance  and  the  capacity  of  the  antenna  with  which 
it  is  to  be  employed  is  known,  but  for  receiving  work  with  an  aerial  having  a  natural  period 
of  450  meters,  it  may  be  28  inches  in  length  by  3l/2  inches  in  diameter,  wound  closely  with 


RECEIVERS  FOR  UNDAMPED  OSCILLATIONS  OR  WAVES. 


283 


No.  22  S.  S.  C.  wire.  A  tap-off  should  be  taken  every  inch  and  led  to  the  contacts  of  a 
multiple  point  switch.  The  leads  to  the  switch  from  the  tap-off  should  not  be  "bunched" 
but  spaced  as  far  as  possible. 

226.    Oscillating  Vacuum  Valve  Detector  Circuits  of  the  U.  S.  Navy. — 

A  modified  vacuum  valve  circuit  for  the  reception  of  undamped  oscillations  by  the  produc- 
tion of  "beats,"  is  described  by  Dr.  Austin  in  the  Proceedings  of  the  Institute  of  Radio 
Engineers.  It  is  a  duplicate  of  the  circuit  developed  at  the  Naval  Laboratory  and  employed 
at  Naval  stations  throughout  the  United  States  and  outlying  possessions. 

It  will  be  noted  from  the  diagram,  Fig.  295,  that  a  certain  similarity  exists  between  this 
circuit  and  the  one  described  in  the  previous  paragraph,  both  generating  a  "beat"  current 
by  the  superposition  of  locally  generated  oscillations  upon  those  of  the  incoming  wave. 

It  is  to  be  noted,  first,  that  the  two  terminals  of  the  secondary  winding  are  connected  to 
the  grid  and  plate  of  the  vacuum  valve  rather  than  to  the  grid  and  filament,  the  more  usual 
connection;  and,  upon  close  examination,  the  secondary  and  tertiary  circuits  are  found  to 
be  electrostatically  and  electro  magnetically  coupled  through  the  condenser  C-5  and  head 
telephone  P  respectively.  And,  as  in  the  former  circuit,  the  incoming  oscillations  and  the 
final  audio-frequent  currents  are  reinforced  by  repeating  them  back  to  the  grid.  Simultane- 
ously, by  adjustment  of  condensers,  C-5,  C-l  and  C-2,  the  valve  generates  oscillations  locally 
at  a  frequency  determined  by  the  constants 
of  the  secondary  and  tertiary  circuits. 

A  novel  feature  of  this  circuit  is  the 
placing  of  the  head  telephone  in  shunt  to 
the  battery,  B-2,  rather  than  in  series,  but 
it  is  claimed  by  Dr.  Austin  that  this  con- 
nection results  in  a  decrease  of  interference 
from  atmospheric  electricity. 

An  important  adjunct  to  this  system  is 
the  so-called  "sensitizing  circuit"  L-4,  C-3. 
It  is  advanced  by  Dr.  Austin  that  this  cir- 
cuit reduces  the  amplitude*  of  the  local 
oscillations  and  therefore  increases  the 
strength  of  the  received  signals  from  three 
to  four  times.  It  also  permits  looser 
coupling  between  the  primary  and  second- 
ary circuits  of  the  system,  thus  cutting 
down  the  interference  of  static. 

A  different  and  more  plausible  explana- 
tion is  advanced  by  Armstrong  for  the 
action  of  the  "sensitizing"  circuit.  Accord- 
ing to  this  investigator,  when  the  grid  cir- 
cuit of  the  vacuum  valve  is  set  into  oscilla- 
tion at  a  frequency  differing  from  the  in- 
coming oscillations,  the  reactance  of  the 
circuit  to  the  local  oscillations  is  zero,  but 
the  reactance  and  therefore  the  impedance 
to  the  incoming  oscillations  is  considerable 
and  results  in  a  loss  of  energy  (due  to  the  dissimilarity  of  the  wave  length  adjustment  of 
the  primary  and  secondary  circuits). 

Now  the  effect  of  coupling  the  "sensitizing"  circuits  to  the  grid  circuit,  is  to  give  the 
latter  two  frequencies  of  oscillation  differing  by  1,000  to  1,500  cycles.  By  adjusting  the  system 
to  oscillate  at  one  of  the  resulting  frequencies  and  having  the  other  "coincide  with  that  of  the 
incoming  oscillations,  zero  reactance  is  obtained  and  the  signal  strength  accordingly  increased. 

Owing  to  the  severe  discharges  of  atmospheric  electricity  experienced  at  the  U.  S.  Naval 
Stations,  it  has  been  found  necessary  to  connect  the  grid  of  the  valve  to  earth  through  a 
small  variable  condenser;  for,  otherwise,  communication  between  the  naval  stations  could 
not  be  maintained. 

No  explanation  of  the  function  of  the  audio-frequency  impedance  coils,  with  the  iron 
cores,  has  been  advanced. 

It    is    needless    to    say    that    many    other    circuits,    when    used    in    connection    with    the 


Fig.    295 — Beat   Receiver   Circuit   as    Employed   at  the 
U.   S.  Naval  Station. 


*This   is   an    important   consideration    in   the   operation    of  the   heterodyne,    namely,   that   the   amplitude 
of  the   locally  generated   oscillations  be  carefully   regulated   for  maximum  response. 


284 


PRACTICAL   WIRELESS   TELEGRAPHY. 


purpose 


P.ADIOFREQUENCY 
TRANSFORMER 


RADIO  FREQUENCY 
TRANSFORMER 


Fig.   296 — Showing  the   General   Electric  Company's  Cascade   Connection   of 
Electron  Relays. 


vacuum  valves,  have  been  found  suitable  to  the  reception  of  undamped  oscillations;  in  fact, 
the  majority  of  these  circuits  are  equally  well  adapted  for  the  reception  of  damped  oscilla- 
tions, but,  as  mentioned  previously,  the  normal  note  of  a  spark  transmitter  is  distorted  when 
the  maximum  amplification  is  obtained. 

(a)  G.  E.  Amplification  Relays.     One  system  of  connecting  magnifying  relays  for  the 
•pose  of  first  increasing  the  amplitude  of  the  oscillations  of  radio-frequency  is  set  forth  in 

Fig.  296,  which  is  a 
method  of  connection 
particularly  suited  to  the 
pliotron  oscillators  of  the 
General  Electric  Com- 
pany. 

It  is  to  be  observed 
that  the  usual  grid  con- 
denser is  excluded  from 
the  circuit  of  all  valves 
except  the  last  one. 
The  oscillations  of 
radio-frequency  are  re- 
peated from  one  bulb  to 
the  other,  and  in  the  last 
bulb  they  are  made  audi- 
ble by  being  rectified, 
e.  g.,  each  group  of  oscillations  places  a  charge  in  the  grid  condenser  which  has  a  relaying 
effect  upon  the  battery  circuit  of  bulb  No.  3  (over  the  duration  of  a  wave  train),  causing  a 
single  sound  for  each  spark  of  the  distant  transmitter. 

The  foregoing  explains  the  functioning  of  this  circuit  during  the  reception  of  damped 
oscillations,  and  it  should  be  mentioned  that  this  system  for  repeating  radio-frequencies 
affords  a  marked  degree  of  selectivity.  For  example,  if  each  of  the  tuned  circuits  is  adjusted 
to  give  response  to  the  incoming  signal  which  has  20  times  the  strength  of  an  interfering 
signal  and  furthermore  the  desired  signal  is  amplified  ten  times  by  each  oscillation  valve,  it  is 
self-evident  that  the  interfering  signal  will  practically  disappear  in  the  circuit  of  the  last  bulb. 
The  circuit  of  Fig.  296  can  be  made  responsive  to  undamped  oscillations  if  the  tertiary 
circuit  of  valve  No.  3  is  shunted  by  an  inductance  and  capacity.  With  this  connection  the 
amplified  incoming  oscillations  interact  with  the  locally  generated  oscillations  and  produce 
a  beat  current  in  the  usual  manner. 

The  student  should  note  that  the  valves  in  Fig.  296  have  a  source  of  direct  current 
electromotive  force  in  series  with  the  grid  to  fix  definitely  the  potential  of  the  latter  in 
respect  to  the  filament  and  also  that  the  third  valve  has  a  source  of  E.  M.  F.  in  shunt  to  the 
grid  condenser  to  prevent  the  accumulation  of  excessive  potentials  on  the  latter.  This  pre- 
caution is  usually  necessary  with  extremely  highly  exhausted  vacuum  valves. 

The  transformers,  P-l  and  P-2,  shown  in  the  diagram,  are  radio-frequency  transformers 
shunted  by  variable  condensers  of  small  capacity  for  obtaining  conditions  of  resonance. 

227.  The  Goldschmidt  Tone  Wheel. — The  mode  of  operation  of  this 
detector  during  the  reception  of  undamped  oscillations  will  be  more  readily  understood  by 
reviewing  certain  facts  in  connection  with  the  functioning  of  the  ordinary  tikker.  When 
the  tikker  interrupts  (periodically)  the  secondary  circuit  of  a  receiving  transformer  tuned 
to  a  distant  transmitter,  the  interruptions  do  not  take  place  in  synchrony  with  the  incoming 
oscillations.  The  telephone  condenser,  therefore,  receives  an  irregular  charge  which,  at  one 
instant,  may  be  taken  at  the  peak  of  the  charge  accumulated  in  the  secondary  condenser, 
and,  at  another  instant,  at  a  charge  of  lesser  value,  the  final  result  being  the  production  of  a 
note  in  the  telephone  of  irregular  pitch  which  is  not  highly  pleasing  to  the  ear. 

The  Goldschmidt  tone  wheel  overcomes  this  defect  by  converting  the  incoming  oscillations 
into  an  audio-frequent  current.  To  illustrate  its  mode  of  operation  we  may  assume 
that  signals  are  being  received  from  the  high-power  station  at  Tuckerton,  New  Jersey,  the 
normal  wave  length  of  which  is  7,400  meters  corresponding  to  an  oscillation  frequency  of 
40,540  cycles  per  second.  Now,  if  a  specially  constructed  mechanical  circuit  interrupter 
giving  40,540  interruptions  per  second  is  connected  in  the  receiver  circuit  in  place  of  the 
usual  tikker,  the  positive  half,  let  us  say,  of  each  successive  cycle  will  be  interrupted  at  the 
peak,  such  as  A,  B,  C,  D,  E,  etc.,  Fig.  297,  and  the  current  of  successive  negative  halves 
only  will  be  admitted  to  the  receiving  telephone.  There  will  then  pass  per  second  through 


RECEIVERS  FOR  UNDAMPED  OSCILLATIONS  OR  WAVES. 


285 


the  receiving  telephone  40,540  pulses  of  direct  current,  but,  owing  to  their  rapid  rate,  these 

pulses  do  not  individually  affect  the  receiver  diaphragm,  except  when  the  current  is  turned 

on  and  off. 

If  the  circuit  interrupter  be  driven  to  give,  for  example,  39,540  interruptions,  the  result 

will  be  that     indicated  in  the  curves,  Fig.  298a   (not  scaled)   where  alternation  A  is  inter- 
rupted directly  at  the  peak,  al- 

*  B  c  D  E         .       f  ternation  B   further  down   the 

peak,  alternation  C  still  further 
down,  and  so  on. 

Now,  when  a  positive  alter- 
nation is  interrupted  at  the 
peak,  the  full  amount  of  nega- 
tive current  is  admitted  to  the 
head  telephone  and  when  a 
negative  alternation  is  inter- 
rupted at  the  peak,  the  full 
amount  of  the  positive  current 


Fig.    297— Effect    of    the    Goldschmidt    Tone    Wheel    When    Driven 
Synchronously. 


is  admitted  to  the  telephone  (note  also  Fig.  298b).    As  the  positive  alternation  is  interrupted 

at  points  off  the  peak,  a  smaller  amount  of  negative  current  and  a  larger  amount  of  positive 

current  will  be  admitted  and  the  net  result  of  this  will  be  a  decrease  of  telephone  current 

as  the  zero  axis  (Fig.  298a)  is  approached. 

When  the  interruption  of  the  incoming  oscillation  takes  place  at  the  zero  axis,  equal 

amounts  of  positive  and  negative  current  (G  and  G-l,  Fig.  298a)   will  be  admitted  to  the 

telephone  and  there  will 
be  no  deflection  of  the 
telephone  diaphragm.  As 
the  interruptions  con- 
tinue beyond  this  point, 
positive  current  of  grad- 
ually increasing  strength 
will  be  admitted  to  the 
telephone  until  the  full 

A'       B        c'       o'       E'       F        6'  amount  of  positive  cur- 

Fig.  298a— Effect  of  the  Tone  Wheel  When  Driven  As-synchronously.          rent  flows.    There  is  thus 

seen  to  flow  through  the 

telephone,  an  audio-frequent  current  which  rises,  falls  and  reverses  uniformly  at  a  frequency 

which  is  .the  numerical  difference  of  the  interruptions  of  the  tone  wheel  and  the  frequency  of 

the  incoming  oscillations. 

The  resulting  note  in  the  telephone  is  found  to  have  a  musical  pitch  which  can  be  varied 

over  a  range  equivalent  to  frequencies  lying  between  200  and  3,000  cycles  per  second. 

In  the  diagram,  Fig.  298b   (not  scaled),  the  interruptions  of  the  positive  and  negative 

incoming  oscillations  are  shown  by  the  curve  A,  D,  and  the  amplitude  of  the  telephone  current 

by  curve  B,  C.  The  peak 
of  the  dotted  curve  is  the 
point  at  which  a  positive 
alternation  is  interrupted 
at  the  peak,  and,  as 
shown  by  the  peak  B,  the 
audio-frequent  current  is 
negative  and  at  a  maxi- 
mum. The  peak  D  is  the 

Fig.   298b— Showing   Superposed  Audio-frequent   Current.  P.°int    ™here    .a    negative 

alternation  is  interrupted 

at  the  peak  and  the  telephone  current,  as  shown  by  peak  C,  is  seen  to  be  positive  and  at  a 
maximum.  At  all  other  points  on  the  curve,  the  strength  of  the  periodic  telephone  current 
will  be  less  varying  as  the  relative  amounts  of  positive  and  negative  current  admitted  to 
the  telephone  by  the  tone  wheel. 

To  produce  a  note  of  audio-frequency  from  the  Tuckerton  transmitter  at  its  present  wave 
length,  a  toothed  wheel  interrupter  having  800  contact  segments  revolving  approximately  at 
3,000  revolutions  per  minute  would  be  required.  This  would  interrupt  the  receiver  circuit 
39,450  times  per  second,  which  would  give  a  note  equal  to  the  pitch  of  1,000  per  second. 


286 


PRACTICAL  WIRELESS  TELEGRAPHY. 


The  tone  wheel  is  frequently  termed  a  frequency  transformer  because  it  converts  me- 
chanically the  oscillations  of  radio-frequency  into  current  of  audio-frequency.  It  is  superior 
to  the  ordinary  tikker,  first,  because  it  produces  a  musical  pitch  so  highly  desirable  for 
aurul  reception,  and,  second,  it  converts  more  energy  of  the  incoming  oscillations  into  useful 
sound  than  does  the  tikker. 

A  photograph  of  the  Goldschmidt  tone  wheel  appears  in  Fig.  299.  The  commutator  disc 
of  this  detector  is  driven  at  speeds  approximating  4,000  revolutions  per  minute. 


Fig.    299— The    Tone    Wheel    Installed    at   the    Tuckerton    Station. 

228.  Marconi  System  for  Reception  of  Undamped  Oscillations. — The 
diagram  of  connections,  Fig.  300,  covers  the  systems  for  the  reception  of  un- 
damped oscillations,  developed  by  Signor  Marconi  and  H.  J.  Round. 


Ttkphont 
Fig.    300— Marconi's    System    for   the    Reception    of   Undamped   Oscillations. 

The  circuit  in  general  contains  the  fundamentals  of  the  Marconi  balanced  crystal  re- 
ceiver, and,  as  explained  in  a  previous  paragraph,  if  crystal  No.  1  and  crystal  No.  2  are 
adjusted  independently  to  the  maximum  degree  of  sensitiveness,  they  will  produce  practically 
equal  and  opposite  effects,  resulting  in  the  production  of  no  sound  in  the  head  telephone.  1 
potentiometer  P-2,  let  us  say,  is  adjusted  so  that  crystal  No.  2  is  in  a  non-conductive  state 
to  signals,  they  will  be  received  at  moderate  strength  on  crystal  No.  1.  However,  very  strong 


RECEIVERS  FOR  UNDAMPED  OSCILLATIONS  OR  WAVES.  287 

impulses  of  static  or  atmospheric  electricity  will  cause  both  crystals  to  become  equally  con- 
ductive resulting  in  the  almost  complete  elimination  of  this  interference. 

To  make  this  apparatus  receptive  to  undamped  oscillations,  a  circuit  J,  C-l,  C-2,  C-3,  C-4, 
is  provided,  which  is  acted  upon  by  the  buzzer  energized  inductance  B.  The  voltage  set  up  in 
J  alters  the  conductivity  of  the  crystals  in  the  following  manner :  Crystals  No.  1  and  No.  2 
are  adjusted  to  oppose  one  another  in  the  usual  manner,  but  the  potentiometer  voltage  is 
adjusted  so  that  only  very  strong  signals  can  be  recorded.  Now  if  circuit  B  is  adjusted  to  a 
very  short  wave,  the  electromotive  force  induced  by  the  buzzer  acts  for  short  intervals  on 
the  two  crystals,  making  them  conductive  for  a  brief  period,  and  thereby  releasing  the  energy 
stored  up  within  the  condenser  K,  in  accordance.  Condenser  K  is,  of  course,  constantly 
charged  by  the  undamped  oscillations  induced  in  the  receiving  aerial.  If  the  groups  of 
waves  from  the  buzzer  have  a  slightly  different  period  or  a  submultiple  of  the  initial  fre- 
quency of  the  incoming  oscillations,  an  audio-frequent  current  will  flow  in  the  head  telephone. 
For  example,  if  the  frequency  of  the  incoming  oscillations  is  50,000  cycles  per  second  and 
the  buzzer  produces  4,900  groups  of  oscillations  per  second,  the  result  will  be  a  musical  tone 
in  the  head  telephones  having  a  pitch  equal  to  1,000  per  second. 

These  circuits  have  been  used  by  Marconi  and  his  assistants  for  the  reception  of  signals 
over  very  great  distances. 


PART  XVI. 

MARCONI  TRANSOCEANIC 
RADIO  TELEGRAPHY. 

229.  MARCONI  DEVELOPMENT  AND  GENERAL  CONSIDERATIONS. 

230.  MARCONI'S  DUPLEX  SYSTEM.     231.  THE  BALANCING  OUT 
AERIAL.     232.  GLACE  BAY-CLIFDEN  STATIONS.     233.  MARCONI 
DIRECTIONAL    AERIAL.     234.  MARCONI     TRANSOCEANIC     STA- 
TIONS.     235.  MARCONI    TUBULAR    MASTS.      236.  RADIO-FRE- 
QUENCY CIRCUITS  OF  THE  DAMPED  WAVE  TRANSMITTERS.     237. 
OTHER  U.  S.  HIGH  POWER  STATIONS.     238.  LONG  DISTANCE 
RECEIVING    SETS.     239.    CONDENSED    LIST    OF    HIGH    POWER 
STATIONS. 


229.  Marconi  Development  and  General  Considerations. — The  greater 
share  of  credit  for  establishing  radio  communication  over  extremely  great  dis- 
tances, is  directly  due  to  the  energetic  and  painstaking  experimenters,  Marconi  and 
his  staff  of  engineers.  Realizing  at  an  early  date  that  the  commercial  utility  of 

wireless  telegraphy  was  not 
confined  to  ship  to  shore 
communication  alone,  Sig- 
nor  Marconi  began  the  con- 
struction of  powerful  sta- 
tions on  both  sides  of  the 
Atlantic  as  early  as  1901. 

The  first  high  power  sta- 
tions for  trans-oceanic  com- 
munication were  established  at 
Poldhu,  Cornwall,  England,  and 
South  Wellsfleet,  Cape  Cod, 
U.  S.  A.,  but,  as  was  antici- 
pated, these  stations  were  re- 
quired solely  for  the  dispatch  of 
traffic  to  ships  at  great  dis- 
tances at  sea  and,  therefore, 
other  sites  were  selected  as 
soon  as  possible  at  Glace  Bay,  Nova  Scotia,  and  Clifden,  Ireland. 

A  number  of  experiments  were  carried  out  at  this  station,  seemingly  insurmountable  ob- 
stacles were  overcome  and,  by  continued  application  and  research,  an  efficient  transmitter 
and  receiver  were  finally  developed,  which  could  be  depended  Upon  for  24-hour  communica- 
tion between  these  two  points. 

An  unfortunate  fire  at  Glace  Bay,  Nova  Scotia,  in  1909,  delayed  the  further  use  of  these 
stations  for  several  months,  but  in  the  process  of  rebuilding  a  more  efficient  station  was  con- 
structed, which  has  been  in  commercial  service  continually  since  that  time. 

The  work  of  designing  wireless  stations  such  as  the  Marconi  Company  have 
erected  for  trans-oceanic  work  is  essentially  an  enlargement  of  the  work  in  con- 
nection with  the  sets  of  only  a  few  kilowatts  used  for  ship  and  shore  stations. 
The  engineer  cannot,  however,  sit  down  with  a  slide  rule  and  multiply  the  figures 


Fig.  300a — Early  Marconi  Station  at  Glace  Bay,  Nova  Scotia. 


MARCONI  TRANSOCEANIC  RADIO,  TELEGRAPHY.  289 

for  his  calculations  relating  to  the  small  stations  by  a  hundred  or  so,  and  have  as 
a  result  a  station  that  will  work  over  a  long  range.  Many  problems  present  them- 
selves in  a  large  radio  installation  which  require  much  care  and  experimentation 
adequately  to  cope  with  the  requirements  of  high  power  transmitters  and  which, 
in  smaller  sets,  are  of  so  little  importance  as  to  be  practically  negligible. 

The  reader,  to  assure  himself  that  the  construction  of  the  high  power  stations  is  an 
engineering  proposition  of  the  highest  order,  need  only  consult  the  accompanying  photo- 
graphs showing  in  a  very  small  part,  the  apparatus  in  use  at  certain  of  the  Marconi  Sta- 
tions in  the  United  States  and  abroad.  Not  only  were  the  technical  problems  encountered 
in  their  construction  broad  in  their  scope  and  somewhat  unusual,  but  the  erection  of  masts 
and  building,  the  transportation  of  materials,  the  furnishing  of  water  supply,  sanitation, 
the  provision  for  power,  the  housing  of  employees,  etc.,  each  brought  their  individual  prob- 
lem which  required  most  painstaking  calculations  and,  in  some  cases,  the  provision  of 
new  means  to  overcome. 

It  is  not  adequately  possible  in  the  space  at  our  disposal  to  go  over  the  details  of  con- 
struction of  the  various  Marconi  high  power  stations,  but  a  brief  description  of  the  equip- 
ment will  be  given,  together  with  such  additional  information  as  may  make  clear  the 
general  plan  of  these  stations  and  the  method  of  operation. 

As  explained  in  Chapter  XIV,  both  continuous  and  discontinuous  wave  systems 
are  employed  for  long  distance  radio  communication,  and  we  should  add  that 
either  system  has  its  advantages.  It  cannot  be  denied  that  undamped  oscillations 
give  better  syntonic  effects  at  the  receiver,  but  almost  equal  selectivity  can  be  ob- 
tained by  feebly  damped  oscillations. 

Generally,  undamped  wave  transmitting  apparatus  requires  very  careful-adjust- 
ment and  is  not  continuously  operative  over  extended  periods  without  very  frequent 
attention,  whereas  the  spark  transmitters,  even  of  500  K.  W.  capacity,  can  func- 
tion over  very  extended  periods  with  only  nominal  care. 

The  general  trend  of  development  in  the  United  States,  however,  seems  to  be 
in  the  direction  of  undamped  wave  transmitters  which,  more  and  more,  are  being 
put  to  commercial  use.  The  final  result  of  experimentation  in  this  direction  cannot 
be  foretold,  but  the  results  already  obtained  seem  to  indicate  that  even  short  wave 
undamped  oscillation  generators  may  shortly  be  available. 

230.  Marconi's  Duplex  System. — A  complete  resume  of  the  series  of  ex- 
periments which  made  trans-oceanic  communication  possible,  would  in  itself  con- 
stitute a  volume  of  no  small  proportions,  but  one  of  the  unique  features  of  the 
Marconi  high  power  stations,  Which  has  doubled  the  volume  of  traffic  that  can  be 
handled  from  continent  to  continent,  with  a  given  set  of  stations  and  has  con- 
tributed to  the  prevention  of  traffic  interruption  by  outside  interference,  is  Mar- 
coni's Duplex  System,  combined  with  his  "balancing  out"  aerial. 

The  mode  of  operation  of  his  Duplex  System  can  perhaps  be  better  understood 
by  citing  a  particular  example  of  commercial  use.  As  already  mentioned,  the  Mar- 
coni high  power  station  at  Glace  Bay,  Nova  Scotia,  transmits  to  another  high 
power  station  at  Clifden,  Ireland,  but  in  order  that  traffic  may  be  dispatched  in 
"both  directions  simultaneously  additional  receiving  stations  have  been  erected  on 
both  sides  of  the  Atlantic,  a  few  miles  distant  from  either  transmitting  station.  This 
separation  of  stations  would  not  in  itself  prevent  interference  between  the  trans- 
mitter and  receiver,  but  by  combining  this  plan  with  the  balancing  out  aerials  inter- 
ference is  wholly  prevented. 

The  transmitter  at  Glace  Bay,  Nova  Scotia,  is  approximately  30  miles  north  of  the 
receiving  station  located  at  Louisburg,  Nova  Scotia,  while  the  receiving  station  for  the 
opposite  side  is  at  Letterfrack,  Ireland,  approximately  12  miles  north  of  the  transmitter 
at  Clifden,  Ireland. 

With  this  separation  of  transmitter  and  receiver,  Glace  Bay  station,  for  instance,  may 
dispatch  messages  to  Letterfrack  freely  and  not  interfere  with  Louisburg  station  receiving 
from  Clifden,  Ireland1.  Not  only  may  this  system  be  used  for  simultaneous  transmission 
in  both  directions,  but  it  can  be  used  as  a  "break-in"  system  as  well,  permitting  the  receiv- 


290 


PRACTICAL    WIRELESS    TELEGRAPHY. 


ing  operator  to  break  in  on  the  sending  operator,  asking  for  the  repetition  of  a  word, 
a  letter  or  a  sentence  if  necessary. 

Now  that  the  general  plan  of  the  Duplex  System  has  been  explained,  the  reader  should 
refer  to  the  diagram,  Eig.  301,  showing  in  a  more  detailed  manner  the  operation  of  the 
Duplex  System  in  a  specific  instance,  viz.,  as  employed  on  the  Glace  Bay-Clifden  route. 

The  150  K.  \V.  transmitter  at  Glace  Bay,  Nova  Scotia,  connected  to  aerial  A,  B,  is 
designated  as  station  Xo.  1.  Operated  at  the  wave  length  of  8,000  meters,  this  station 
transmits  to  Letterfrack,  Ireland,  direct,  designated  as  station  No.  2. 

The  transmitter  at  Clifden,  station  No.  4,  transmits  to  Louisburg  station  No.  3  at  the 
wave  length  of  5,500  meters. 


5  5000  METER  WA\/E 


GLACE  BAV    N  s 

TRANSMITTED       STATION  N» 


LETTtRFRACK    IRELAND 
RECEIVER     RATION  N?  2 


5500     METER  WAVE 


LOUISBURG    N    5 
RECEIVER     STATION 


CLIFDE.N    IRELAND 
TRANSMITTER       STATION   N 


Fig.    301 — Marconi's    Duplex    High    Power    Transmitter    Scheme    With    Balancing    Out   Aerials. 

Stations  No.  1  and  No.  3  are  connected  by  wire  line,  similarly,  stations  No.  2  and  No.  4. 
The  sending  operator  for  station  No.  1  is  located  at  station  No.  3,  the  transmitter  at  No.  1 
being  actuated  by  a  manually  operated  key  or  automatic  sender,  the  circuit  of  which  ex- 
tends over  the  wire  line  connecting  the  two  stations. 

Similar  arrangements  are  in-  use  on  the  opposite  side  of  the  Atlantic,  the  transmitter 
at  Clifden,  Ireland,  being  controlled  by  an  operator  stationed  at  Letterfrack.  Both  the 
transmitting  and  the  receiving  operators  may  thus  work  side  by  side,  traffic  being  dis- 
patched freely  in  both  directions.  But  to  positively  insure  against  interruptions  of  service, 
the  transmitting  stations  are  equipped  with  receiving  sets,  to  be  employed  in  event  of  a 
breakdown  at  the  regularly  appointed  receiving  stations.* 

*The  general  scheme  and  mode  of  operation  of  the  high  power  stations  herein  described  is  that 
originally  planned.  This  is  not  necessarily  the  way  these  stations  are  now  operated.  Changed  conditions 
dre  to  the  European  situation  have  required  some  re-arrangement  of  the  original  plan  and  method. 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


291 


231.  The  Balancing  Out  Aerial. — It  will  be   well  to  understand  that  al- 
though transmitting  stations  No.   1   and  No.  4,  Fig.  301,  operate  at  wave  lengths  of  8,000 
meters  and  5,500  meters  respectively,  some  of  the  energy  of  the  8,000  meter  wave  is  apt  to 
induce  oscillations  of  its  own  frequency  in  the  receiving  aerial,  C,  D,  station  No.  3,  when  the 
latter  is  adjusted  to  the  wave  length  of  5,500  meters. 

In  the  same  manner,  station  No.  4  would  induce  oscillations  in  the  receiving  aerial  at 
No.  3,  setting  up  no  small  amount  of  interference,  were  it  not  for  the  use  of  the  balancing 
out  aerials,  E,  F,  and  E1  F1  at  stations  No.  3  and  No.  2,  respectively. 

These  balancing  aerials  are  generally  located  at  a  right  angle  to  the  main  receiving 
aerial  in  such  a  way  as  to  receive  the  maximum  induction  from  the  nearby  transmitter  and 
the  minimum  induction  from  the  far  distant  transmitter. 

The  circuit  of  the  balancing  aerial  is  tuned  to  the  interfering  wave  and  inductively  op- 
posed to  the  interfering  oscillations  induced  in  the  main  receiving  aerial  at  the  secondary 
winding.  Tlien  by  proper  adjustment  of  coupling  and  phase  relation  of  currents,  the  energy 
induced  by  the  interfering  wave  is  electro- 
magnetically  balanced  out,  leaving  the 
main  receiver  aerial  free  to  receive  from 
the  far  distant  transmitter.* 

In  the  diagram,  Fig.  301,  aerial  C,  D, 
for  instance,  at  station  No.  3,  is  favor- 
ably disposed  for  the  reception  of  signals 
from  aerial  A1,  B',  and  the  balancing  aerial 
E,  F,  receives  the  maximum  induction 
from  A,  B,  and  the  minimum  from  A1  B1. 

Similarly,  balancing  aerial  E1,  F1,  re- 
ceives the  maximum  induction  from  A1, 
B1,  and  the  minimum  induction  from  A, 
B.  If  oscillations  be  induced  in  C,  D, 
by  the  8,000  meter  wave  from  A,  B,  and 
thereby  cause  interference  to  the  recep- 
tion of  signals  at  the  wave  length  of  5,500 
meters  from  A1,  B1,  the  balancing  aerial 
E,  F,  is  tuned  to  8,000  meters  and  its 
circuit  inductively  opposed  to  the  secon- 
dary of  the  tuner  connected  to  C,  D. 
Proper  adjustment  of  coupling  and  de- 
crement permits  the  oscillations  (induced 
by  the  8,000-meter  wave  in  each)  to  be 
brought  opposite  in  phase,  and  the  inter- 
fering signals  thus  balanced  out,  signals 
being  received  all  the  while  from  A1,  B1. 

In  practice,  this  System  has  been  found         F'g-    302 — Fundamental   Circuit   of   the    Glace   Bay  Type 

to  work  well,  preventing  the  transmitter 

and  receiver  from  interfering  with  each  other,  although  separated  no  more  than  8  or  10  miles. 

232.  Glace   Bay-Clifden   Stations. — We   have   already   intimated   that   the 
installation  and  the  subsequent  successful  operation  of  the  Glace  Bay  and  Clifden 
high  power  stations  opened  up  a  new  era  in  long-distance  communication.    In  view 
of  the  pioneer  work  performed  at  these  two  points,  it  may  be  of  interest  to  de- 
scribe fundamentally  the  transmitters  in  use. 

Contrary  to  previous  installations,  the  condensers  were  charged  by  14,000  volts  direct 
current,  the  source  of  which  was  a  battery  of  6,000  storage  cells  connected  in  series.  These 
batteries  in  turn  are  charged  by  three  5,000-volt  generators  (A,  B,  C,  Fig.  302),  of  very 
special  construction,  connected  in  series  and  then  to  the  terminals  of  the  battery. 

As  will  be  seen  from  the  fundamental  diagram,  Fig.  302,  the  energy  stored  up  in  the 
condenser  C-3  is  discharged  through  the  usual  oscillation  transformer  and  Marconi's  studded 
disc  discharger. 

This  discharger  consists  of  a  large  steel  disc  several  feet  in  diameter,  with  sparking 
electrodes  mounted  thereon  and1  which  pass  closely  to  the  fixed  electrodes  B,  B. 


DI5C 
DISCHARGER 


See    Section    I    Appendix. 


292  PRACTICAL   WIRELESS   TELEGRAPHY. 

The  electrodes  B,  B,  in  reality  revolve  slowly,  and  therefore  present  constantly  cooled 
surfaces  to  the  path  of  the  spark  discharge.  Protective  chokes  C-l  and  C-2  protect  the 
generator  from  the  discharge  of  the  condenser. 

The  principal  advantage  of  the  disc  discharger  is  the  quenching  effect  obtained  when  the 
disc  is  driven  at  a  certain  velocity.  This  prevents  the  re-transference  of  energy  to  the 
spark  gap  circuit,  and  in  consequence,  the  antenna  oscillations  have  a  decrement  as  low  as 
.03  per  complete  cycle.  In  addition  a  spark  or  group  note  of  distinct  musical  pitch  is  obtained 
which  has  been  found  extremely  desirable  for  aural  reception  through  atmospheric  electricity. 

The  high  potential  condensers  at  both  of  these  stations  are  of  unusual  construction  and 
dimensions.  Consisting  of  a  number  of  large  steel  plates  suspended  from  the  ceiling  on 
special  insulators,  they  are  separated  sufficiently  to  prevent  the  spark  discharging  between 
plates,  and  the  required  capacity  is  found  in  connecting  a  large  number  of  such  plates  in 
parallel. 

Beyond  the  size  of  this  condenser,  the  oscillation  transformer  comes  in  for  attention, 
principally  because  of  its  dimensions.  The  primary  winding  has  but  two  turns,  consisting 
of  a  specially  constructed  cable,  one  foot  in  diameter.  Owing  to  the  large  amounts  of 
power  in  use,  a  cable  of  these  dimensions  is  positively  required,  and  good  surface  conduc- 
tivity is  among  the  chief  considerations. 

Signalling  is  not  effected  by  interruption  of  the  primary  circuit  as  in  small-sized  Marconi 
transmitters.  In  these  sets,  specially  constructed  high  voltage  relay  switches  (S-2),  in  turn 
controlled  by  a  small  telegraph  key  and  source  of  D.  C.  current,  interrupt  the  high  voltage 
circuit  from  the  battery  to  the  condenser.  Arcing  is  prevented  by  forcing  an  extra  heavy 
air  blast  on  the  contact  points  by  specially  designed  motor  blowers. 

The  receiving  apparatus  at  these  stations  may  either  consist  of  a  Marconi  balanced  crys- 
tal receiver,  employing  the  carborundum  rectifier,  or  a  set  of  Fleming  vacuum  tubes  of 
special  design,  type  and  construction.  Through  a  set  of  microphonic  relays,  connected  in 
cascade,  the  signals  may  be  amplified  considerably  and  either  deciphered  in  an  ordinary  ear 
telephone  or  the  pulsations  of  current  may  be  indented  on  the  wax  records  of  a  dictaphone. 

233.  Marcohii  Directional  Aerial. — The  great  success  of  Signer  Marconi's 
Trans-oceanic  system  is  in  no  small  measure  due  to  the  use  of  the  horizontal  direc- 
tional aerial*     Fully  convinced  by  a  series  of  quantitative  experiments  that  the 
flat  top  aerials  radiate  more  freely  in  the  direction  opposite  to  which  the  free  end 
points,  particularly  if  the  length  of  the  flat  top  exceeds  the  length  of  the  vertical 
portion  by  four  or  five  times,  Signor  Marconi  decided  that  the  adoption  of  this 
aerial  would  not  only  permit  the  transmission  of  messages  over  great  distances 
with  small  powers  but  also  on  account  of  its  directional  properties  would  prevent 
a  considerable  amount  of  interference  to  the  operation  of  other  stations. 

In  the  same  series  of  experiments,  it  was  determined  that  a  flat  top  aerial  receives  with 
greater  intensity  when  the  free  end  points  in  the  direction  opposite  to  the  free  end  of  the 
transmitter  aerial.  Irrespective  of  its  selective  directional  properties,  a  horizontal  aerial 
of  given  capacity  and  inductance  for  any  required  wave  length,  is  less  expensive  to  erect 
than  a  vertical  aerial  of  similar  electrical  dimensions ;  hence,  from  this  consideration  alone, 
the  flat  top  aerial  is  the  one  that  would  be  adopted. 

In  order  to  radiate  the  energy  of  a  300  K.  W.  transmitter,  the  aerial  should  have  a 
fundamental  wave  length  of  at  least  6,000  meters ;  in  fact  the  greatest  distances  are  covered 
when  such  aerials  radiate  near  to  their  fundamental  wave  length. 

The  great  Marconi  station  at  New  Brunswick,  New  Jersey,  U.  S.  A.,  for  example,  has 
an  aerial  of  32  wires  connected  in  parallel,  5,000  feet  in  length.  The  aerial  is  supported  by 
12  tubular  steel  masts,  400  feet  in  height,  arranged  in  two  rows  of  six  each.  The  funda- 
mental wave  length  is  approximately  8,000  meters,  but  the  initial  transmitting  experiments 
were  carried  on  at  the  wave  length  of  15,000  meters. 

The  receiving  aerial  for  this  station  at  Belmar,  New  Jersey,  consists  of  two  wires  6,000 
feet  in  length,  suspended  on  six  tubular  masts,  400  feet  in  height.  The  aerial  has  a  general 
direction  favorable  for  reception  from  the  giant  transmitting  station  at  Carnarvon,  Wales. 

234.  Marconi  Transoceanic  Stations. — By  far  the  greater  number  of  high 
power  radio  stations  here  and  abroad  have  been  designed  and  erected  by  the 

*An    explanation    of    the    cause    of   the    unsymmetrical    radiation    of   an    inverted   L    aerial    appears   in 
Page   167  of  Fleming's  Elementary  Manual   of  Radio  Telegraphy. 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY.  293 

Marconi  Company.  In  fact,  their  stations  only  have  maintained  a  continuous 
operating  schedule  from  day  to  day,  from  continent  to  continent.  Individual  con- 
cerns may  have  carried  out  spectacular  experiments  here  and  there,  but  nothing 
has  been  evolved  by  them  that  would  tend  to  make  long-distance  communication 
a  success  commercially.  The  mere  fact  that  a  message  may,  for  instance,  be  sent 
across  the  ocean  by  a  low-powered  transmitter  and  received  on  a  small  aerial  at 
certain  hours  of  the  day  is  no  indication  that  such  equipment  could  be  used  for 
continuous  24-hour  service,  because  experiment  reveals  that  very  large  powers 
are  required  for  continuous  operation,  when  the  sender  and  receiver  are  3,000 
miles  apart. 

Those  who  are  familiar  with  the  great  globe-girdling  scheme  of  the  Marconi  Company 
cannot  help  but  be  impressed  with  the  stupendous  undertaking  involved  in  the  construction 
of  their  high  power  stations,  for  not  only  is  the  task  of  d'esigning  the  apparatus,  buildings 
and  power  machinery  one  of  extraordinary  undertaking,  but  the  actual  installation  of  such 
has,  in  many  instances,  called  for  painstaking  labors  and  effort  largely  due  to  the  location, 
the  nature  of  the  soil,  and  the  topography  of  the  surrounding  country. 

In  view  of  the  universal  interest  manifested  by  students  of  radio  in  behalf  of  high  power 
radio  stations  of  the  Marconi  Company,  a  brief  description  of  their  equipment  will  be  pre- 
sented, together  with  such  additional  information,  as  will  make  clear  the  general  plan  and 
mode  of  operation.  First  let  it  be  explained  that  although  these  stations  could  all  be 
made  intercommunicative,  it  is  more  usual  to  construct  a  pair  of  stations  to  cover  a  specific 
route  or  to  join  together  two  continents  only. 

With  the  idea  in  view  of  showing  which  of  these  stations  was  intended  for 
communication  with  the  other,  they  shall  be  grouped  into  ''radio  circuits"  or  routes, 
as  follows : 

CIRCUIT  NO.  1. 

Stations     sep-f  Glace  Ba£  *£?  Scotia"' transmits  t0"- Lett*rfrack>   ]  Stations     sep- 
arated     about^     Louisb          No;a   Scotia...  receives  f  rom . . .  Clif  den,    fara£d    ..about 
Ireland. 

Since  the  apparatus  for  the  Glace  Bay  station  has  been  very  briefly  described  in  para- 
graphs 274  and  275,  it  will  not  be  gone  over  again,  except  to  mention  that  the  Duplex  Sys- 
tem has  been  installed  and  thoroughly  tested.  Because  these  two  stations  established  the 
first  successful  trans-oceanic  commercial  radio  service,  they  are  purposely  grouped  at  the 
head  of  the  list. 

CIRCUIT  NO.  2. 

C   New  Brunswick,  N.  J.,  U.  S.  A transmits  to "1  Qfofl- 

ab^J  Towrn?  Wales,  Great  Britain,  fe"8  .££ 

50  miles        [    Belmar'  Waje'sU'  S'  A"  ' '""""  fr°m- '  -C^"3™"'  j       62  miles. 

The  transmitting  station  at  New  Brunswick  is  of  300  K.  W.  capacity  and  can  be  operated 
at  various  wave  lengths  from  7,000  to  15,000  meters.  Power  is  taken  in  the  station  from  a 
commercial  power  house  at  1,100  volts,  3  phase  60  cycle  alternating  current,  stepped  down 
to  440  volts  and  led  to  the  terminals  of  a  60  cycle,  440-volt  3  phase  550  H.  P.  motor,  which 
drives  a  300  K.  W.  120  cycle  generator. 

The  current  is  led  from  the  generators  to  a  bank  of  high  voltage  transformers,  the  sec- 
ondaries of  which  may  be  connected  in  series  or  in  parallel  according  to  the  power  required. 

In  the  usual  manner  the  current  from  these  transformers  charges  a  large  bank  of  high 
voltage  oil  plate  condensers  which,  in  turn,  discharge  through  an  oscillation  transformer 
and  rotary  disc  discharger  of  uncommon  proportions.  As  in  the  Glace  Bay  Station,  the 
circuit  from  the  transformer  secondaries  to  the  condenser  is  interrupted  by  a  specially 
designed  set  of  high  tension  relay  keys  which,  in  turn,  are  actuated  by  a  small  sending  key 
and  a  source  of  direct  current. 

Arcing  at  the  contacts  of  the  main  signalling  key  is  prevented  by  a  heavy  blast  of  air 
forced  directly  at  the  contact  points  by  specially  designed  motor  blowers.  The  advantages 


294 


PRACTICAL  WIRELESS  TELEGRAPHY. 


derived  in  interrupting  of  the  high  voltage  current,  lies  in  that  it  permits  300  K.  W.  to  be 
handled  at  various  speeds  of  transmission  up  to  100  words  per  minute  without  error. 

A  more  detailed  description  of  certain  apparatus  of  the  circuits  of  radio  frequency  for 
the  Xew  Brunswick  station  and  others  -with  like  equipment  (damped  wave  apparatus)  will 
be  given  in  paragraph  236. 


Fig.    303 — Power   House    of   the   Trans-Atlantic   Marconi    Station    at    Carnarvon,   Wales. 

The  transmitting  aerial  at  the  New  Brunswick  station  is  of  the  inverted  L  type,  consist- 
ing of  32  wires  with  a  flat  top  approximately  5,000  feet  in  length.  It  is  supported  on  two 
rows  of  steel  tubular  masts  (6  masts  in  each  row),  which  are  approximately  400  feet  in 
height.  The  two  rows  of  masts  are  separated  about  250  feet. 

The  receiving  aerial  at  Belmar  is  6,000  feet  in  length,  consisting  of  2  wires  supported 
on  six  tubular  masts,  each  400  feet  in  height. 


Fig.   304 — Motor  Blowers  at  Carnarvon  Station. 

The  transmitter  at  Carnarvon,   Wales,   is   substantially  a  duplicate   of   the   New   Bruns- 
wick  transmitter,   the    source   of   power   being   a   300   K.   W.,    150   cycle   motor  generator 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


295 


with  step-up  transformers,  oil  condensers,  etc.  Of  late,  a  150  K.  W.  timed  spark  discharger, 
excited  hy  5,000  volts  continuous  current,  has  been  employed  as  well,  and  with  which  par- 
ticularly successful  results  have  been  obtained.  Operated  usually  at  the  wave  length  of 
10,000  meters,  daylight  communication  has  been  established  with  the  U.  S.  A.,  the  strength 
of  signals  being  equal  to  that  obtained  from  other  foreign  stations  of  much  greater  power. 
(For  a  more  detailed  explanation  of  timed  spark  discharger  sets  see  paragraph  219.) 

Some  idea  of  the  construction  of  the  Marconi  high  power  stations  may  be  ob- 
tained from  the  following  description: 

The  power  house  of  the  transmitting  section  of  the  Wales  Transoceanic  station  at  Car- 
narvon, Wales,  is  shown  in  Fig.  303,  wherein  the  aerial  and  ground  leads  of  the  great 
antenna  which  transmits  messages  to  the  Belmar,  New  Jersey,  station  appear  prominently 
in  the  foreground.  This  building  measures  approximately  100  feet  by  83  feet,  and  is  divided 
into  three  sections,  known  as  the  main  machinery  hall,  the  annex,  and  the  extension.  The 


Fig.  305 — Three  Hundred  Kilowatt  150  Cycle  Generators  at  Carnarvon  Station. 

transmitting  sets,  switchboard,  transformer  room,  stores,  offices  and  emergency  operating 
room  are  located  in  the  main  machinery  hall.  The  auxiliary  plant  is  placed  in  the  annex, 
consisting  essentially  of  D.  C.  generators,  electrically  driven  blowers  and  ventilating  fans, 
and  some  small  motor  generator  sets  used  in  the  signalling  circuit.  An  office  for  the  engi- 
neers and  a  fitting  shop  are  also  provided  for  in  the  annex.  The  extension  is  devoted 
entirely  to  experimental  apparatus.  All  trans-Atlantic  wireless  messages  transmitted  from 
this  station  will  be  handled  automatically  from  London,  through  the  receiving  section  at 
Towyn,  sixty-two  miles  away,  and  received  at  Belmar  for  automatic  transmission  to  New 
York.  This  station  is  therefore  of  great  interest  to  Americans  as  the  communicating  links 
with  the  New  Jersey  stations  in  the  Marconi  globe-girdling  chain. 

In  Fig.  304  are  shown  the  blowers  which  furnish  air  under  considerable  pressure,  to  blow 
out  the  spark  at  the  disc  discharger  and  keep  the  disc  elements  cooled.  These  are  also  used 
to  blow  out  the  sparks  at  the  signalling  switches  which  relay  the  dots  and  dashes  to  the 
aerial  wires. 

In  Fig.  305,  the  300  K.  W.  150  cycle  motor  generators  at  the  Carnarvon  station  are  shown 
as  installed  ready  for  use.  In  the  photograph,  Fig.  306,  are  shown  the  signalling  motor  gen- 
erators and  the  disc  motor  starters  at  Carnarvon.  One  of  each  is  a  spare.  The  signalling 
motor  generator  supplies  current  to  work  the  high  speed  relay  switches  through  which  the 
station  is  enabled  to  transmit  from  a  distant  operating  station  at  the  rate  of  100  words  a 
minute.  The  motor  starters  shown  on  the  right  control  the  75  H.  P.  motors,  which  drive  the 


296 


PRACTICAL   WIRELESS   TELEGRAPHY. 


disc  discharger  when  it  is  disconnected  from  the  main  generator  for  as-synchronous  working. 
Photograph,  Fig.  307,  gives  a  view  of  the  high  voltage  transformers  and  primary  induc- 
tances.    All  the  current  from  the  generators  passes  through  the  transformers,   where  it  is 

stepped  up  to  a  voltage 

_  sufficient    to    charge    the 

condensers.  The  low 
frequency  inductances 
shown  to  the  right  of  the 
drawing  permit  a  large 
range  of  adjustment  in 
the  primary  power  cir- 
cuits, thereby  permitting 
the  radiated  energy  to  be 
controlled  in  accordance 
with  the  requirements. 
Fig.  308  shows  the 
switchboard  at  the  New 
Brunswick,  New  Jersey, 
station.  This  board  con- 
trols the  generator  cir- 
cuits, blower  machinery 
and  all  controlling  appli- 
ances within  the  station. 
The  receiving  station 
at  Belmar,  New  Jersey, 

is  completely  equipped  with  a  Marconi  balanced  crystal  receiving  set,  Brown  amplifying  relays, 
a  balancing  out  aerial  for  eliminating  interference,  dictaphone  receivers,  and  a  set  of 
telegraphic  instruments  for  connection  with  the  land  line  telegraph  and  telephone  companies. 
These  transmitting  and  receiving  stations  not  only  have  the  necessary  buildings  for  the  hous- 
ing of  the  apparatus,  but  hotels  and  individual  dwellings  are  supplied  for  the  employees  as 
well. 


Fig.   306 — Special   Signalling  Generators  at  Carnarvon  Station. 


Stations 
arated 

30  miles. 


*ep7    f Marion,     Mass. 
)out      Chatham,  Mass. 


CIRCUIT  NO.  3. 

.transmits. . .  Stavanger,*     Norway, 
.receives  from.  .  .Naerboe,  Norway. 


Stations      sep- 
arated     about 
30  miles. 


At  the  writing  of  this  volume,  this  group  of  stations  are  under  construction  and  very 
nearly  completed.  They  will  be  used  for  24-hour  commercial  working  and  will  permit  com- 
munication with  Northern  European  countries,  independent  of  all  existing  routes,  obviating 
the  necessity  for  a  number  of  intermediate  relay  points. 

The  transmitter  at  Marion  will  be  a  150  K.  W.  Marconi  timed-spark  continuous  wave 
generator,  energized  by  a  300  K.  W.  5,000  volt  D.  C.  generator.  The  transmitter  at  Stavanger 
will  be  substantially  a  duplicate,  with  ultimate  capacity  of  300  K.  W.  Since  they  have  been 
found  the  most  economical  and  practical  for  the  purpose,  the  aerials  at«  these  stations  are 
supported  by  tubular  steel  masts.  As  usual,  the  stations  are  constructed  for  Duplex  working, 
Marion  and  Chatham  as  well  as  Stavanger  and  Naerboe  stations,  being  con- 
nected together  by  land  line  control.  These  stations  will  be  placed  in  commercial  operation 
within  a  very  short  time. 

CIRCUIT  NO.  4. 

04.04.;,™  r  Bolinas,       Calif ....  transmits       to...Koko       Head,  1    , 

abou1;  J  Hawaiian  Islands,  I    Statl°ns      *eP- 

>OUt  1  Manshalls,    Calif. ...  receives    from. .  .Kahuku,    Ha-  f  arated      about 

25  miles. 


40  miles. 


Stations     sep- 
arated     about 
60  miles. 


waiian  Islands. 

CIRCUIT  NO.  5. 

r  Kahuku,    Hawaiian  Islands . . .  transmits    to ...  Funa- 

bashi,  Japan, 
j  Koko    Head,    Hawaiian  Islands . . .  receives    from .  . . 

Funabashi,  Japan. 


*Station    is   located    at    Hinna. 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


297 


Because  the  transmitter  at  Kahuku  is  duplexed  for  simultaneous  transmission  to  Japan 
and  the  U.  S.  A.,  the  two  circuits,  No.  4  and  No.  5,  have  been  grouped  together.  Beginning 
with  the  Bolinas  station,  the  transmitter  is  of  300  K.  W.  capacity,  current  for  its  operation 
being  supplied  by  duplicate  500  H.  P.  steam  turbine  driven  generators  delivering  current  at 
180  cycles  per  second.  In  the  usual  manner,  this  current  is  stepped  up  by  closed  core  trans- 
formers to  approximately  50,000  volts  and  employed  to  charge  a  bank  of  high  voltage  oil 
plate  condensers.  Although  normally  operated  at  from  75  to  150  K.  W.  the  full  300  K.  W. 
can  be  employed  whenever  necessary. 

The  aerial  for  receiving  from  Bolinas,  Cal.,  is  nearly  a  mile  in  length  erected  on  two 
rows  of  tubular  steel  masts  in  the  usual  manner.  The  receiving  aerial  at  Marshalls,  Cali- 
fornia, has  7  masts,  each  of  which  are  330  feet  in  height. 

The  receiving  station  at  Koko  Head,  Hawaiian  Islands,  has  two  distinct  receiving  aerials, 
together  with  balancing  out  aerials,  one  being  employed  for  reception  from  Bolinas,  Calif., 
and  the  other  from  Funabashi,  Japan. 


Fig.    307 — Bank    of    High    Voltage    Trans-     Fig-    308 — The    Switchboard    of    the    New    Brunswick 
formers    at    Carnarvon    Station.  High   Power  Transoceanic   Station. 


The  aerial  for  receiving  from  Bolinas  extends  southwestward  from  the  operating  house 
and  is  carried  on  five  330  feet  masts  to  an  anchorage  on  the  beach.  The  aerial  for 
reception  from  Japan  extends  from  the  operating  room  almost  due  east.  The  first 
two  masts  for  this  aerial  are  of  the  standard  sectional  type  430  feet  in  height;  the  first  is  on 
level  ground  and  the  second  is  on  the  hillside.  From  this  point  the  aerial  makes  a  long  span 
of  over  2,000  feet  to  the  top  edge  of  Koko  Head  (an  extinct  volcano)  at  an  elevation  of  1,194 
feet  above  the  sea  level ;  here  there  is  not  enough  room  to  erect  a  sectional  mast,  only  about  40 
square  feet  being  available  for  a  self-supporting  structural  tower  150  feet  in  height.  The  tail 
end  anchorage  for  this  aerial  is  far  down  the  volcano  on  the  inside  of  the  crater.  The  balanc- 
ing aerial,  which  is  employed  for  both  receiving  aerials,  is  erected  on  self-supporting  towers, 
each  of  which  is  100  feet  high.  All  this  will  be  clear  from  the  diagram,  Fig.  309,  wherein  a 


298 


PRACTICAL  WIRELESS  TELEGRAPHY. 


complete  lay-out  of  the  receiving  station  at  Koko  Head  appears  showing  the  relative  posi- 
tions of  the  balancing  out  aerial,  the  location  of  buildings,  etc.  It  is  to  be  noted  that  the 
balancing  out  aerial  is  5,700  feet  in  length,  and  is  arranged  to  be  favorable  for  the  absorp- 
tion of  energy  from  the  two  transmitting  stations  at  Kahuku. 

Because  it  is  duplexed  for  the  simultaneous  transmission  of  messages  to  Japan  and  the 
United  States,  especial  interest  attaches  to  the  Marconi  station  at  Kahuku,  Island  of  Oahu. 
Hawaiian  Islands.  Not  only  is  this  station  fitted  with  two  300  kilowatt  transmitting  sets  but 
a  third  emergency  set  has  been  installed  as  well,  which  in  event  of  breakdown  can  be  con- 
nected either  to  the  Japan  or  the  United  States  aerial. 

The  general  layout  of  the  antennae  and  buildings  at  Kahuku  is  shown  in  the  diagram, 
Fig.  310,  wherein  it  will  be  noted  that  the  free  end  of  these  aerials  point  in  a  direction 


RECEIVING     STATION 

KOKO    HEAD 
ISLAND        or        OAHU 


Fig.    309 — Plan    and   General    Layout    of    the    Receiving    Aerials    at    Koko    Head,    Hawaiian    Islands. 

favorable  for  the  particular  continent  with  which  communication  is  to  be  established,  being 
designated  as  the  "Japan"  aerial  and  the  "San  Francisco"  aerial. 

From  the  power  house  as  a  center,  the  California  transmitting  aerial  extends  southwest- 
ward,  supported  by  twelve  masts,  325  feet  in  height;  the  Japan  aerial  extends  to  the  south- 
east, supported  by  fourteen  masts,  475  feet  in  height.  These  masts  are  the  largest  that  have 
been  yet  constructed  on  the  Marconi  system  of  sectional  cylinders.  The  power  house  con- 
sists of  boiler  room,  engine  room  and  condenser  room.  The  boilers  are  oil-fired  and  will 
feed  three  500  H.  P.  turbines,  which  drive  the  special  300  K.  W.  alternators  and  Marconi 
disc  discharger. 

The  necessary  condenser  capacity  for  all  three  transmitting  sets  is  "found  in  768  large 
oil  tank  type  condensers,  which  are  conveniently  arranged  for  uniform  distribution  of  current 
to  all  connecting  bus  bars. 

The  automatic  sending  and  receiving  apparatus  plays  an  important  part  in  the  service 
between  the  Occident  and  the  Orient.  The  sending  machine  consists  of  a  Wheatstone  auto- 
matic transmitter  and  special  perforator,  which  makes  possible  the  transmission  of  more 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


299 


than  100  words  a  minute.  Under  the  automatic  system,  ten  or  100  messages  can  be  filed 
at  the  same  time  at  the  office  of  the  Marconi  Company  in  Honolulu.  They  will  be  distributed 
among  the  necessary  number  of  operators  and  the  dots  and  dashes  punched  in  a  paper 
tape  by  a  typewriter  perforator  This  tape  is  fed  into  an  automatic  sender  and  the  signals 
conveyed  by  land  line  to  Kahuku,  where  the  dots  and  dashes  actuate  a  high  voltage  sending 
key,  automatically  energizing  the  aerial  instantaneously  with  the  feeding  of  the  tape  in  the 
station,  thirty  miles  or  more  away.  At  the  transmitting  station  the  dots  and  dashes  operate 
the  magnets  of  the  high  power  sending  key  in  the  main  energy  circuits  and  the  signals  are 


TRANSMITTING    STATION 

KAHUKU 
ISLAND    or  OAHU 


LOG AT  i 

Fig.   310 — General   Plans  of  Transmitting  Aerials   at    Marconi    Station,    Kahuku,    Hawaiian    Islands. 

flashed  to  whichever  destination  the  message  calls  for — either  Marshalls  or  Funabashi.  If 
the  message  is  destined  for  Marshalls  it  will  be  received  on  a  specially  constructed  dicta- 
phone machine,  each  cylinder,  as  soon  as  it  is  indented  with  dots  and  dashes,  being  handed 
to  an  operator,  who  transcribes  it  into  a  typewritten  message  by  means  of  a  reproducing 
dictaphone  machine,  running  at  normal  speed. 

The  Imperial  Japanese  Government  station  at  Funabashi,  Japan,  is  equipped  with  a 
200  K.  W.  quench  spark  transmitter,  but  complete  details  of  the  equipment  are  not  yet 
available. 

235.  Marconi  Tubular  Masts. — One  of  the  most  interesting  features  of  the 
original  construction  work  at  the  Marconi  high  power  stations  was  the  erection  of  the  steel 
tubular  masts,  the  successive  stages  of  erection  being  shown  in  Figs.  311,  312,  313,  314  and 
315.  The  mast  is  made  up  of  steel  cylinders  (Fig.  311),  constructed  in  quarter  sections, 
flanged  vertically  and  horizontally  and  secured  together  by  bolts  stayed  with  steel  cables. 
These  stand  in  a  concrete  foundation.  Surmounting  the  main  steel  column  was  a  wooden 
top  mast,  the  lower  part  of  which  is  squared  and  set  in  square  openings  in  the  plates  between 


300 


PRACTICAL  WIRELESS  TELEGRAPHY. 


Fig.     311— Showing    Steel     Semi-Cylinders    for 
the    Marconi    Tubular    Mast. 


the  steel  cylinders.    The  hoisting  arms  attached 
to  the  upper  end  were  fitted  with  blocks  and 
hoisting  cables.     Attached  to  these  arms  were 
chain  hoists  supporting  a  square  wooden  cage 
(Fig.  312)  for  the  workmen,  which  was  lowered 
or  raised  as  the  demands  of  the  work  required 
while  the  sections  were  being  bolted  together. 
The  wooden   topmast  was  the   keynote   of 
this   novel    system   of   construction,    operating 
like  a  man  who  pulls  himself  up  by  his  boot- 
straps.    The  lower  half  of  this  topmast  is  of 
square  section  and  is  guided  by  a  square  hole 
in  the  diaphragm  plates  between  each  section. 
The  topmast  was  fitted  with  a  set  of  hoisting 
arms   which  carried  blocks  through  which  reaved  the  material  hoisting  ropes.     A  square 
wooden  cage  was  suspended  from  the  hoisting  arms  by  four  chain  hoists  so  that  the  work- 
men in  it  could  move  themselves  up  and  down  to  bolt 
the  sections   together.      This   is   more  clearly  shown  in 
Fig.  314. 

Assume  that  two  cylinders  have  been  bolted  to  the 
bed  plate,  the  mast  rising  through  the  center.  The  sec- 
tions of  the  third  cylinder  were  raised  by  a  steam  winch 
and  bolted  in  place  by  the  workmen.  Then  a  heavy 
flexible  steel  rope  was  temporarily  anchored  at  the  top 
of  this  last  cylinder.  Attached  to  the  top  of  the  steel 
section,  this  cable  led  down  inside  the  cylinders  and 
around  a  wheel  in  the  foot  of  the  wooden  topmast;  then 
it  was  carried  up  again  on  the  other  side  and  around  a 
sheave  to  the  top  of  the  steel,  thence  to  the  winch.  By 
pulling  on  this  rope  the  topmast  was  raised  the  length  of 
one  cylinder  and  pinned  through  holes  in  both  steel  and 
wooden  masts.  With  the  addition  of  a  new  cylinder,  the 
topmast  was  raised  again,  the  pin  supporting  it  until  this 
was  brought  about  (Fig.  313).  The  stays  were  attached 
at  the  required  points  as  the  erection  of  the  mast 
progressed. 

The  stays,  by  means  of  which  each  mast  is  supported, 
are  made  of  heavy  plough  steel  cable,  possessing  great 
tensile  strength.    For  each  mast  thousands  of  feet  of  this 
cable  were  used,  great  care  being  taken  to  see  that  the 
elastic  extension  of  these  stays  was  not  so  great  as  to  result  in  the  vibration  of  the  mast 
during  heavy  winds.     It  was  essential  to  break  each  stay  into  short  lengths  connected  with 
great  porcelain  insulators  in  order  that  the  electrical  energy  might 
not  be  absorbed,  led  to  the  earth  by  the  stays  and  lost  for  purpose 
of  wireless  operation.  For  all  connections  at  the  masts,  insulators 
and  anchorages,  special  bridge  sockets  were  designed.    This  did 
/fff  S  away  with  the  necessity  for  splicing  and  permitted  a  perfect  and 

straight  pull,  thereby  developing  the  strength  of  the  cable.  Heavy 
concrete  blocks  were  used  as  anchorages  for  the  stays.  The 
completed  mast  is  shown  in  Fig.  315. 

In  addition  to  the  antennae  stretched  between  the  masts,  great 
quantities  of  wire  were  placed  in  the  ground  about  the  stations  in 
order  to  provide  an  efficient  earthing  system  or  ground  connec- 
tion. Told  in  brief,  a  circle  of  zinc  plates  is  buried  in  a  trench, 
bolted  together  and  jointed  to  the  wireless  circuits  of  the  power 
house  by  copper  wires.  Wires  radiate  from  the  zinc  plates  in  the 
ground  to  a  set  of  outer  plates,  from  which  extend  another  set 
of  earth  wires  placed  in  trenches  running  the  full  length  of  the 
aerial.  The  general  scheme  for  the  earth  connection  is  shown  in 
Fig.  320. 

236.  Radio-Frequency    Circuits    of    the    Damped 
stages  of  Construction!      Wave  Transmitters. — A  description  will  now  be  given 


Fig.  312 — Showing  Workmens'  Cage 
Which  is  Carried  to  the  Top 
During  the  Process  of  Erection. 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


301 


of  certain  apparatus  of  the  radio-frequency  circuits  of  the  high  power  transmitting  stations 
employing  apparatus  for  the  production  of  damped  waves. 

As  in  the  small  station,  the  principal  circuits  are  simple,  but  it  is  easily  seen  that  when 
about  three  hundred  kilowatts  are  to  be  carried  in  the  circuits  and  turned  into  high  frequency 
currents,  the  problem  must  be  handled  with  care.    Some  of  the  special  arrangements  made  to 
use   this   large   amount   of   current   safely 
and  efficiently  may  be  of  interest  to  the 
student. 

The  closed  oscillating  circuit  is  of  in- 
terest because  of  the  size  and  construction 
of  its  various  elements.  The  discharger  is 
of  the  rotating-disc  type,  and  is  directly 
connected  to  the  shaft  of  the  alternator. 
The  foundation  upon  which  it  rests  is  a 
solid  block  of  concrete  weighing  about 
seven  tons,  supported  by  piles  of  insulat- 
ing compound  in  such  a  manner  that  the 
framework  of  the  discharger  is  well  pro- 
tected. The  coupling  for  the  alternator 
is  also  insulated,  the  only  live  part  being 
the  discs  which  serve  as  the  poles  between 
which  the  studs  pass.  The  spark  dis- 
charge is  well  quenched  by  a  blast  of  air 
under  high  pressure.  In  order  that  the 


Fig.  314— Showing  the  Cage  and 
the  Top  Mast  Several  Hun- 
dred Feet  from  Earth.  Fig.  315— Completed  Mast  (Guys  Not  Shown). 

phase  relation  between  the  alternator  current  and  the  spark  may  be  varied  to  the  most 
advantageous  point,  the  alternator  is  built  with  the  armature  frame,  the  machine  being  of  the 
rotating  field  type,  mounted  in  a  secondary  outside  frame,  with  a  track  machined  in  its 
inside  circumference.  On  this  track  the  armature  frame  may  be  rotated  through  an  arc  equal 
to  the  angle  between  the  poles,  by  means  of  a  hand  wheel.  Thus  the  spark  may  be  made  to 
discharge  the  condenser  at  any  desired  point  of  the  sine  wave.  The  dischargers  are  set  in 
sound  muffling  rooms  and  the  leads  run  up  through  the  ceiling  to  the  oscillation  trans- 
formers mounted  on  the  floor  above.  These  rooms  are  exhausted  by  motor-driven  blowers 


302 


PRACTICAL   WIRELESS   TELEGRAPHY. 


r^-i 


J1L 


SIDE:  vie.w 

OSCILLATIOM     TRA 


Fig.    316 — Side    View    of    300    Kilowatt    Oscillation    Transformer. 

of  capacity  sufficient  to  keep  the  air  fresh  and  to  draw  out  the  air,  which  is  admitted  by 
the  high  pressure  blast  used  for  quenching  the  spark. 

One  of  the  precautions  taken  to  provide  an  uninterrupted  service  is  to  duplicate  any  and 
all  pieces  of  apparatus  which  are  at  all  likely  to  be  disabled.  Thus,  two  generators  and  two 
disc  dischargers  are  supplied  for  each  station,  and  a  ready  means  provided  to  connect  either 
one  to  the  bus  and  the  oscillation  transformer. 


PLAN   view 
OSCILLATION 


Fig.    317 — Plan    View    300    Kilowatt    Oscillation    Transformer    Showing    High    Tension    Bus-Bar. 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


303 


As  the  oscillation  transformers  are  worthy  of  note,  a  sketch  of  them  accompanies  this 
Chapter  (Figs.  316,  317  and  318).  It  will  be  seen  that  the  coils  are  mounted  on  a  horizontal 
axis  and  supported  on  a  wooden  frame.  The  primary  coil  is  in  series  in  the  closed  circuit. 
It  consists  of  four  turns,  each  turn  being  almost  a  complete  ring  of  about  345  degrees.  These 
turns  are  connected  by  heavy  copper  plates,  making  the  equivalent  of  a  helix  of  four  turns. 
The  helix  is  five  feet  in  diameter  (See  Fig.  318),  and  the  section  of  the  turns  one  foot  in 
diameter.  The  turns  are  made  of  a  wooden  former,  upon  which  the  conductors  are  grouped. 
They  follow  along  the  length  of  the  former,  but  have  a  long  pitch  in  a  spiral  direction 
laid  upon  it  in  such  a  way  that  the  length  of  each  wire  is  the  same  starting  from  one  connect- 
ing plate  to  the  next,  that  is,  through  the  arc  of  345  degrees.  All  the  conductors  are,  in  them- 
selves, made  up  of  stranded  insulated  copper  wire. 

The  secondary  coil  of  the  oscillation  transformer  is  wound  on  a  wooden  former,  com- 
posed of  two  end  plates  with  wooden  rods,  upon  which  are  mounted  porcelain  spools.  On 
these  spools  the  con- 
ductor is  wound;  it 
consists  of  fourteen 
turns  of  a  specially 
built  high  frequency 
cable.  The  cable  is 
made  by  winding 
thirty-six  small  con- 
ductors around  a 
hemp  center  core, 
each  conductor  being 
made  of  seven  double 
cotton  covered  wires. 
There  is  a  cotton 
cover  over  the  whole 
cable,  which  is  about 
three  inches  in  diam- 
eter. In  mounting 
this  secondary  coil 
on  the  frame,  the 
barn-door  type  of 
roller  bearings  is  em- 
ployed. This  makes  a 
device  that  can  be 
constructed  totally  of 


Fig. 


318 — The     300     Kilowatt     Oscillation    Transformer     Installed     at     the 
Marconi  High   Power   Station   at   Kahuku,   Hawaiian    Islands. 


wood,  and  for  close  adjustment  a  wooden  hand  screw  is  used,  which  will  clamp  the  coil  on 
the  supporting  bar  of  the  frame,  as  well  as  furnish  the  means  for  adjustment. 

From  the  oscillation  transformer,  the  buses  (Fig.  317)  are  led  to  the  center  of  the  con- 
denser room,  and  as  the  transformer  is  on  the  second  floor  of  the  powerhouse,  the  main 
leads  are  carried  horizontally  about  fourteen  feet  above  the  condenser  room  floor.  The 
condenser  bank  is  divided  into  four  groups,  each  group  being  fed  from  the  overhead  buses. 
Each  group  is  then  subdivided  into  four  sections  of  twenty-four  tanks.  They  are  connected 
in  parallel  series,  three  tanks  being  in  each  series  and  eight  rows  of  three  tanks  each  in 
parallel  in  each  section. 

From  the  central  point  of  the  room  the  overhead  buses  lead  radially  to  points  directly 
over  the  center  of  each  section.  Here  the  leads  are  carried  down  to  a  level  with  the  con- 
denser tops.  From  this  point  the  sections  are  fed.  The  main  bus  is  made  of  twenty-four 
inch  wide  copper,  bent  in  trough  shape,  the  return  lead  being  supported  under  the  other. 
At  every  point  where  the  leads  divide,  a  smaller  size  conductor  is  used,  and  all  the  leads  are 
so  arranged  that  the  distance  between  them  can  be  adjusted,  in  order  that  the  inductance 
(of  the  leads)  can  be  varied  as  the  requirements  of  the  circuits  demand.  By  using  this 
system  of  distribution,  the  path  of  the  current  from  the  oscillation  transformer  to  any  tank 
in  the  entire  bank  of  three  hundred  and  eighty-four  is  exactly  the  same.  This  is  very  im- 
portant, in  order  that  no  one  tank  will  be  required  to  work  on  a  greater  current  than  any 
other.  Each  section  of  twenty-four  tanks  is  set  on  a  sloping  floor,  and  from  each  section 
runs  an  oil  drain,  under  the  floor,  to  a  single  receiver. 

The  condenser  tanks  at  the  high  power  stations  are  modifications  of  the  Poldhu  type 
shown  in  Fig.  319.  They  consist  of  stoneware  tanks,  thirty  inches  high  by  seventeen  inches 
wide,  by  seven  inches  thick,  The  design  was  adopted  for  the  purpose  of  keeping  in  view 


304 


PRACTICAL   WIRELESS   TELEGRAPHY. 


the  necessity  of  the  quick  and  easy  replacement  of  plates.  All  the  elements  of  a  tank  can  be 
lifted  out  bodily  and  new  plates  of  glass  readily  inserted.  A  fibre  sling  carries  thirty-one 
glass  plates  and  twenty-nine  zinc  plates,  every  alternate  zinc  plate  being  connected  to  the 
buses.  .  ,  '  |.  i 

As  will  be  seen  in  the  sketch,  the  plates  are  connected  to  the  terminals  by  flexible  strips, 
which  compensate  for  any  inequalities  in  the  depth  of  the  tanks.  The  covers  are  thoroughly 
oil-sealed,  and  the  whole  tank  is  filled  with  a  high  grade  mineral  oil.  As  the  slightest  trace 
of  moisture  in  the  oil  renders  its  insulating  value  inadequate,  and  as  oil  is  highly  hygroscopic, 
it  is  quite  essential  to  use  only  very  dry  oil.  To  obtain  this,  a  filter  press  is  installed  in  each 
station. 


O 


CONNECTOR, 


CA.OSS   SECTION    Or  TANK.. 


Fig.    319 — Details    of    Condenser    Tanks    for    High    Power    Stations. 

If  any  tank  fails  or  is  broken  down,  the  section  of  which  it  is  a  unit  is  cut  out  of  the 
bank,  and  the  spare  section  cut  in.  Then  a  portable  derrick  is  run  over  the  section,  and  by 
means  of  a  block  and  tackle,  which  it  carries,  the  contents  of  the  tank  are  lifted  out.  To  do 
this,  a  pair  of  tongs  are  run  down  through  the  small  holes  in  the  center  of  the  cover,  and 
the  hooks  inserted  in  the  holes  in  the  fibre  sling.  Thus  the  contents  of  the  tank  can  be  lifted 
out  with  the  cover  attached,  and  a  new  set  of  elements  put  in  very  quickly,  without  lifting 
the  tank  itself.  After  this,  repairs  can  be  made  to  the  elements  at  leisure. 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY. 


305 


EARTHING     SYSTEM 

Hi»w    POWER   TRA.NSMITTINO    STATION 

s=  GENERAL     SCHEME    =«s — 


THESE  EARTH  wine's  EXTEI. 


X 


X 


Fig.  320 — General   Scheme  of  Earth  Connections  for  High  Power  Stations. 


237.  Other  U.  S.  High  Power 
Stations. — In  addition  to  the  Mar- 
coni High  Power  Stations  just  described, 
two  additional  stations  have  been  erected  in 
the  United  States  for  transocean  communi- 
cation, which  are  now  under  the  control  of 
the  United  States  Government.  One  is  lo- 
cated at  Sayville,  Long  Island,  and  the  other 
at  Tuckerton,  New  Jersey.  In  the  original 
installation,  the  Sayville  transmitter  con- 
sisted of  a  100  K..  W.  quenched  spark  set 
operated  at  the  wave  length  of  4,800  meters, 
but  in  the  past  two  years  a  100  K.  W. 
Joly-Arco  undamped  transmitter  has  been 
in  use  and  has  been  operated  at  a  wave 
length  of  10,000  meters.  During  the  favor- 
able hours  of  the  day,  communication  has 
been  established  with  a  corresponding  sta- 
tion located  at  Nauen,  Germany,  the  latter 
station  being  fitted  with  a  powerful  un- 
damped wave  transmitter. 

The  station  at  Tuckerton,  New  Jersey,  is 
equipped  with  a  200  K.  W.  Goldschmidf 
radio-frequency  alternator  and  a  100  K.  W 
Poulsen  arc  type  of  transmitter,  both  of 
which  are  employed  for  communication 
with  a  corresponding  station  at  Hanover, 
Germany,  the  latter  station  being  fitted  with 
a  200  ~K.  W.  Goldschmidt  alternator. 


Fig.   321 — The   Aerial  Tuning  Inductances  at   Kahuku 
High  Power   Station.' 


306 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Both  the  SayvilJe  and  Tnckerton  stations  employ  the  unjbrella  type  of  aerial,  erected  on 
masts  of  great  height.  The  Tuckerton  tower,  in  fact,  is  850  feet  in  height,  while  the  tower 

at  Sayville  is  approximately  550  feet  high, 
and,  as  is  to  he  expected,  the  aerials  sup- 
ported by  these  towers  require  a  great  deal 
of  space  for  erection. 

In  initial  experiments  at  Tuckerton, 
New  Jersey,  the  Goldschmidt  tone  wheel 
was  employed  as  a  receiver,  but  of  late  a 
modified  form  of  the  oscillating  vacuum 
valve  has  been  in  use,  which  gives  increased 
strength  of  signals.  The  receiving  appara- 
tus at  Sayville  is  of  the  vacuum  valve  oscil- 
lating type,  various  circuits  being  used, 
generally  the  type  favored  by  the  particular 
electrician  in  charge. 

238.  Long  Distance  Receiving 
Sets. — Various  types  of  long  dis- 
tance receivers  have  been  developed  by  the 
Marconi's  Wireless  Telegraph  Company, 
Ltd.,  both  for  the  reception  of  damped 
and  undamped  oscillations.  The  high  power 
stations  in  the  United  States  are  fitted  with 
balanced  crystal  or  Fleming  valve  receivers, 
the  local  circuits  of  which  are  connected  to 
3  or  4  microphonic  amplifying  relays,  con- 
nected in  cascade.  By  means  of  these  a 
considerable  amplification  of  the  incoming 
signals  is  obtained,  which,  in  fact,  can 
frequently  be  heard  with  the  receivers 
lying  on  the  table  with  the  transmitting 
station  3,000  miles  distant. 

Marconi  trans-oceanic  receiving  stations 
are  also  completely  equipped  with  dictaphone 
receivers  which  are  arranged  so  that  immediately  one  wax  record  is  indented  with 
signals,  a  second  dictaphone  receiver  is  automatically  connected  in  the  circuit,  and  so  on. 
The  use  of  these  records  permits  traffic  to  be  dispatched  across  the  ocean  at  speeds,  up  to 
75  words  per  minute,  which  is  afterwards  transcribed  on  the  typewriter  by  the  receiving 
operator  at  speeds  varying  from  30  to  40  words  per  minute.  Obviously  this  system  will 
permit  a  greater  amount  of  traffic  to  be  handled  between  two  stations  and  it  will,  therefore, 
be  employed  when  atmospheric  electricity  is  not  too  severe. 

239.  Condensed  List  of  High  Power  Stations. — Some  of  the  long  distance 
transmitting  stations  of  the  Marconi  Company,  their  locations,  systems  and 
power  are  shown  in  the  following  table : 


Fig.   322 — Bank  of  High  Voltage  Transformers 
at  Kahuku. 


Location 
Carnarvon, 
Wales 

System 
Continuous     and     discontinuous 
waves 

Power 
100-300  K.W. 

Wave   Length 
in  Meters 
6,000  to  10,000 

Clifden, 
Ireland 

Discontinuous  waves 

75-150  K.W. 

5,500 

Glace   Bay, 
Nova  Scotia 

Discontinuous  waves 

75-150  K.W. 

8,000 

Stavanger, 
Norway 

Continuous  waves 

150  K.W. 

p 

Marion,  Mass. 

Continuous  waves 

150  K.W. 

? 

New     Brunswick, 
New  Jersey 

Continuous     and     discontinuous 
waves 

150-300  K.W. 

8,000  to 

15,000 

Bolinas, 
California 
Kahuku,  Hawaii 

Discontinuous  waves 
Discontinuous  waves 
Discontinuous 

300  K.W. 
300  K.W. 
.  300  K.W. 

6,000  to 
6,000  to 
6,000  to 

10,000 
12,000 
12,000 

Location 
Sayville, 
Long  Island 

Tuckerton, 
New  Jersey 


MARCONI  TRANSOCEANIC  RADIO  TELEGRAPHY.  307 

OTHER  TRANSOCEANIC  STATIONS. 

System  Power  Wave   Length 

Joly-Arco      (continuous     waves)  100  K.W.  9,400  meters 


Goldschmidt      High      Frequency 
Alternator  (continuous  waves) 


7,400  meters 


arc 


Fig.  323— U.  S.   Naval  Radio  Station  at  Radio,  Va.    (Arlington). 


APPENDIX 

SECTION  A. 
COMPLETE  DATA  FOR  STANDARD  MARINE  COMMERCIAL  SETS. 


Transformer 
Voltage 

Capacity  High 
Voltage  Con- 

Antenna  Current 
(Average) 

Type 

Power  Rating 

Primary 

Secondary       denser  (mfds.) 

600  met.              450  met.   300  met. 

P-4 

2 

K.W. 

500 

cycle 

350-140 

12,500 

.012  (600  met.) 

12  to  17  amp. 

9-13    3 

to  5 

.006  (300  met.) 

P-5          * 

> 

K.W. 

500 

cycle 

350-120 

14,500 

.004 

5^ 

to  8 

5-72 

to  4/ 

E-2           J 

j 

K.W. 

120 

cycle 

300-110 

14,700 

.01 

5  to 

7 

ll/2 

to  3 

Composite 

1 

K.W. 

60 

cycle 

110 

18,000 

.008 

5  to 

7 

!/> 

to  3 

Composite 

2 

K.W. 

240 

cycle 

500 

15,000 

.008 

5/2 

to  8 

1/2 

to  3 

P-9            } 

I 

K.W. 

500 

cycle 

300-110 

15,000 

.002 

3  to 

4 

1 

to  2 

The  foregoing  data  will  familiarize  the  student  with  the  .power  rating,  electrical  dimen- 
sions of  the  oscillation  circuits,  voltages,  current,  etc.,  of  the  various  transmitters  of  the 
American  Marconi  Company,  knowledge  of  which  is  essential  to  the  practical  operator. 

SECTION  B. 
WAVE  LENGTH  RANGE  OF  MARCONI  RECEIVING  TUNERS. 

Type  101 200  to  7,500  meters 

Type  107a   300  to  2,500  meters 

Type  106 200  to  3,500  meters 

Type  112   200  to  2,500  meters 


.0 
.05 
.1 
.2 
.3 
.4 
.5 
.6 
.7 
.8 
.9 
1.0 


SECTION  C. 
CORRECTION  FACTOR  FOR  FORMULA. 

.       27T 

Wave  length  =  —  X  9.5  X  V  L  C7 
K 

La 

Values  of  K  corresponding  to  — 

Li 

Where  L?  =  inductance  of  loading  coil  at  base, 
LA  =  distributed  inductance  of  antenna. 


K 

1.57 

1.51 

1.426 

1.314 

1,219 

1.142 

1.078 

1.022 

.968 

.925 

.892 

,855 


1.5 
2. 
2.5 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 


K 

.735 

.65 

.59 

.545 

.475 

.430 

.40 

.370 

.350 

.325 

.31 


APPENDIX.  309 

SECTION  D. 
1NDUCTIVITY   VALUES    FOR   DIFFERENT   DIELECTRICS. 

Dielectric.  Inductivity  Value,  "K" 

Flint  Glass,  Double  Extra  Dense 10.10 

Flint  Glass,  Very  Dense 7.40 

Flint  Glass,  Light   6.85 

Mint  Glass,  Very  Light  6.57 

Castor  Oil 4.80 

Quartz 4.50 

Porcelain    4.38 

Mica  Sheet,  Pure  4.00  to  8.00 

Glass,  Common   (Radio-Frequency)    4.21 

Glass,  Common   (Low  Frequency)    3.25  to  4.00 

Sperm  Oil  3.02  to  3.09 

Olive  and  Neats-Foot  Oils  3.00  to  3.16 

Shellac 2.74  to  3.60 

Gutta  Percha 2.46  to  4.20 

Sulphur    2.24  to  3.84 

India  Rubber,  Pure  2.22  to  2.497 

Turpentine    2.15  to  2.43 

Hard  Rubber  Ebonite  2.05  to  3.15 

Petroleum     2.03  to  2.42 

Resin    1.77  to  2.55 

Paraffined  Paper  3.65 

Paraffine  Wax   v 1.9936  to  2.32 

Beeswax    1.86 

Paraffine,  Clear   1.68  to  2.32 

Celluloid   1.555 

Manila  Paper 1.50 

Air  at  Ordinary  Pressure,  Standard  1.0000 

SECTION  E. 


Conversion  of  X  =  —  to  X  =  59.6  V  L  C 

N 

If  L  be  expressed  in  henries,  C  in  farads,  R  in  ohms,  and  N  in  cycles  per  second,  Ohm's  law 
for  alternating  current  is  expressed, 

T 


27rNC 
1 

Where  2?rNL—          — =  is  the  expression  for  the  reactance  of  a  circuit  containing  induc- 
tance and  capacity. 

1  E 

If  2^r  N  L  =  -       -  we  have  resonance  and  therefore  I  =  —  (See  paragraphs  45  and  46). 

27r  NC  R 

If  in  any  given  oscillation  circuit, 

2?r  N  L  = then,  evaluating, 


47r2  N2  L  C  =  1 

1 
hence,  N'  = 


310  PRACTICAL   WIRELESS   TELEGRAPHY. 

which  is  the  fundamental  equation  for  the  frequency  of  an  oscillation  circuit.  As  shown  in 
paragraph  91,  if  X,  the  wave  length,  be  expressed  in  meters,  N  in  cycles  per  second  and  V 
the  velocity  of  electrical  waves  =  300,000,000  meters  per  second,  then 

300,000,000 
X— 

N 
1 
But  N= 


27T  V  L  C 
300,000,000 


hence  X  =  — : =  300,000,000  X  2*r  X  V  L  C 

1 


27rV  LC 

The  microhenry  and  the  microfarad  are  more  convenient  units  for  practical  oscillation  cir- 
cuits. Also  1  farad  =  1,000,000  microfarads,  and  1  henry  =  1,000,000  microhenries.  Hence, 
if  the  inductance  and  capacity  are  to  be  expressed  in  such  units  we  must  divide  L  and  C 
by  1,000,000,  then 


J_ 

T  -1    C\C\C\  AAA  1    AAA  AAA 


=2*  X  300,000,000  X 

1,000,000       1,000,000 

If  we  take  1,000,000  out  from  under  the  radical, 
300,000,000 

V  L  C  =  2*-  x  300  X  V  L  C 


1,000,000 

If  L  be  expressed  in  centimeters  and  1  microhenry  =  1,000  centimeters,  then  we  must 
divide  L  by  1,000,  hence, 

X  =  2*-  X  300  X 


1,000 


Taking  out  1,000  under  the  radical  (and  since  V  1,000  =  .31.62) 
6.2832  X  300 

X  = X  V  L  C 

31.62    . 
or  X  =  59.6  VlTC 

SECTION  F. 

The  following  scries  of  questions  bear  directly  and  indirectly  on  the  tc.vt  of  this  volume, 
and  are  considered  as  a  representative  set  for  training  students  to  become  wireless  operators. 
They  were  intended  primarily  to  guide  the  instructor  in  outlining  a  complete  instruction 
course,  but  will  be  valuable  to  the  elementary  student  of  radio  as  well. 

PART   I. 
Magnetism. 

Ques.    (1)  Describe  a  permanent  magnet. 

Ques.   (2)  Name    the    metals    subject    to    magnetic    influence. 

Ques.    (3)  Show   by   diagram   the   resultant  magnetic   field    between  two   north   poles   in   proximity. 

Ques.    (4)  Draw  a   diagram   showing   the   resultant   magnetic   field    between   a   north   and   south   pole. 

Ques.   (5)  What   is   meant   by   the   earth's   magnetism? 

Ques.    (6)  Where   is   the   north  magnetic   pole  located? 

Ques.  (7)  What  is  the  behavior  of  soft  iron  and  hard  steel  under  the  influence  of  a  magnetizing 
force? 

Ques.   (8)  Can    all    bodies    be    magnetized? 

Ques.   (9)  What  is   said  to   be  the  direction   of  the  lines  of  force  in  a  magnet? 


APPENDIX. 


311 


PART  II. 

Elementary  Electricity  and  Simple  Circuits. 

Ques.   (1)     What    is   meant    by   a    "positive"    and  a    "negative"    charge   of   electricity? 

Ques.    (2)      Explain    the    application    of    the    term    "electromotive    force"    to    an    electrical    circuit? 

Ques.   (3)      Name    four    methods    by    which    an    electromotive    force    can    be    generated. 

Ques.    (4)      Explain    briefly    the    actions    taking    place    within    an    ordinary    chemical    electrical    cell. 

Ques.   (5)      Show  by  diagram  a  series    connection  of  electrical  cells. 

Ques.    (6)     Show   by   diagram   a   parallel    connection   of   electrical   cells;    a    series   parallel    connection. 

Ques.    (7)      Define    the    term    resistance    as   applied    to    an   electrical    circuit. 

Ques.    (8)     What    is   meant   by  an   open   circuit;    a   short  circuit;    a   closed   circuit? 

Ques.    (9)     What   is    the  unit  for   current  pressure;    current   strength;    resistance. 

Ques.   (10)      Explain    Ohm's  law   and   apply    it   to   an   electrical    circuit;    work   out   a    simple   problem. 

Ques.    (11)      How    is    the    power    of    a    direct    current    measured? 

Ques.  (12)  Work  out  a  simple  problem  showing  the  joint  resistance  of  three  resistances  of  different 
values  connected  in  parallel. 

Ques.   (13)      Define    a    volt,    ampere,    ohm,    watt    and    give    the    standard   value    of   each. 

Ques.    (14)     What    metals    offer    the    greatest   resistance   to    the    flow    of   an    electrical    current? 

Ques.    (15)     Will  pure  water   conduct   electricity? 

Ques.   (16)     Will    impure    and    salt    water    conduct    electricity? 

Ques.  (17)  The  voltage  of  a  circuit  in  100  volts;  the  resistance  10  ohms;  what  will  be  the 
strength  of  the  current  when  the  circuit  is  closed? 

Ques.    (18)      How   many    amperes   will    a    100   watt    Tungsten    lamp    draw   on   a    120    volt   circuit? 

Ques.  (19)  What  power  is  consumed  in  applying  a  current  at  pressure  of  220  volts  to  a  circuit 
the  resistance  of  which  is  20  amperes? 


PART  III. 

Electromagnetism — Electromagnetic  Induction — Electrical  Measuring  Instruments — The 

Flow  of  Alternating  Current. 

Ques.    (1)      Describe    the    essentials    of    an    electromagnet. 

Show  by   diagram    the   direction   of   the    lines  of   force    about   a    solenoid. 

Show    by    diagram    the    direction    of    the    lines    of    force    about   the    poles    of   a   horse-shoe 


Ques.  (2) 

Ques.  (3) 

magnet. 

Ques.  (4) 

regulated? 

Ques.  (5) 

Ques.  (6) 


How    may    the    strength    of    the    magnet    field    about    the   poles    of    an    electromagnet    be 

Show   by    diagram  the    principle    of   electromagnetic   induction. 
Explain    the    terms    self-induction;    mutual    induction. 


Ques.  (7)  Draw  a  circuit  diagram  of  an  induction  coil  with  a  magnetic  interrupter  and  explain 
fully  its  operation. 

Ques.  (8)  Show  by  diagram  a  step-up  transformer;  a  step-down  transformer;  an  auto  transformer; 
an  air  core  transformer. 

Ques.  (9)  Show  by  diagram  the  construction  and  working  of  a  galvanometer;  a  voltmeter;  an 
ammeter;  a  wattmeter. 

Ques.   (10)      Explain  the  use  of   a  shunt  in  connection  with  an  ammeter. 

Ques.   (11)     Explain    briefly    how    current    is    generated    in    a    dynamo. 

Ques.  (12)  Draw  a  diagram  of  a  shunt-wound  dynamo;  a  series-wound  dynamo;  a  compound-wound 
dynamo. 

Ques.  (13)  Show  by  diagram  the  differences  in  construction  between  a  ring-wound  armature  and 
a  drum-wound  armature. 

Ques.   (14)      How   can   the   voltage   of    a   dynamo   be    increased   or   decreased? 

Ques.   (15)     What  effect  has  an  increase  of  the  speed   of  the  armature  on  the  voltage   of   a  dynamo. 

Ques.    (16)      Show   by    diagram    the   construction    of   a    field   rheostat. 

Ques.  (17)  Explain  by  diagram  the  function  of  a  commutator  in  a  dynamo;  similarly  the  function 
of  the  collector  rings. 

Ques.    (18)      Explain  briefly    how  an   electric    motor   operates. 

Ques.    (19)      State   two    methods   by  which   the  speed  of  a  motor  can    be  increased. 

Ques.    (20)      Show    the    circuits    of    a    differentially    compound-wound    motor. 

Ques.   (21)      How   may    the  frequency    of   a    generator   be    increased? 

Ques.  (22)  What  is  the  frequency,  in  cycles  per  second,  of  the  generators  used  in  commercial 
wireless  telegraphy? 

Ques.  (23)  What  effect  has  a  series  condenser  on  the  flow  of  alternating  current;  the  effect 
of  a  series  inductance. 

Ques.    (24)     What  is   meant    by   a   resonant   circuit? 

Ques.   (25)      Give    Ohm's   law   for    alternating   current    (See    Appendix). 

Ques.   (26)      Explain  the  meaning  of  the  term   "power  factor." 

Ques.   (27)      How    is    the    power    of    an    alternating    current    measured? 

Ques.   (28)      Show    by    diagram   two   practical    electrical    circuits. 

Ques.    (29)     Explain   how   a  fuse  operates. 

Ques.    (30)     What    is    meant    by    the    effective    value    of    an    alternating    current? 

Ques.   (31)     How    is    the    power    output   of    a    generator    increased?      Decreased? 

Ques.   (32)      What    is    meant    by    a    separately   excited    dynamo? 


312 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Ques.  (33)  How  can  you  reduce  the  output  of  an  alternating  current  dynamo  without  reducing 
the  frequency? 

Ques.  (34)  What  is  the  frequency  of  a  30  pole  generator  revolving  at  a  speed  of  2,000  revolutions 
per  minute? 

Ques.   (35)  Define    the   terms   cycle,    frequency   and   amplitude. 

Ques.  (36)  Show  by  diagram  the  difference  between  a  step-up  voltage  transformer  and  a  step- 
down  voltage  transformer. 


PART  IV. 
Motor  Generators  and  an  Automatic  Motor  Starter. 

Ques.   (1)     Draw    a    diagram   of   a    motor    generator    consisting    of   a    differentially-wound    motor    and 
a    simple    shunt-wound    generator. 

Ques.    (2)      Draw    a    circuit    diagram    of    a    simple    shunt-wound    motor    generator. 

Ques.    (3)     Explain    by    diagram    the    Cutler    Hammer    type    of    motor    starter;    similarly    the    General 
Electric    Company's    hand    starter. 

Ques.    (4)      Give    a    complete    circuit    diagram   of   the   automatic    starter   used    in    connection   with    the 
Marconi  2   K.   W.   500   cycle   panel   set. 

Ques.    (5)     Give  a  circuit  diagram  of  a    Y2   K.   W.  automatic  starter  used  in   connection  with  the   */z 
K.   W.    500   cycle    Marconi   panel   set. 

Ques.   (6)      Show    the    use    of    protective    condensers    on    a    motoc    generator;    also    protective    resist- 
ance  rods. 

Ques.    (7)      How    often   must    the    bearings    of    a    motor    generator    be    oiled? 

Ques.   (8)      How   would    you    clean    the    commutator    of    a    motor? 

Ques.   (9)      How   would    you    clean    the    brushes    of    a    motor? 

Ques.   (10)     Show   how   to   locate   an   open   coil   in  a   motor   amature. 

Ques.   (11)      How  would  you  locate  a  short  circuit  in  one  of  the  field  coils  of  the  motor  generator? 

Ques.    (12)      How  would  you  detect  a  "ground"  in  the  circuits  of  a  motor  generator? 

Ques.    (13)      How  would  you   know  that  the    motor  generator  is  running  at  normal  speed? 

Ques.    (14)      How  would   you   repair   a  burned-out  resistance   coil   in  a  motor  starter? 
Show   how    to    repair    a    burned-out   field    rheostat, 
Why    are    high    frequency    generators    used    in    wireless    telegraphy? 


Ques.    (15) 
Ques.   (16) 


Ques.  (17)  Explain    two    different   methods    of    controlling   the    speed    of   a    motor? 

Ques.  (18)  Why    cannot  the   starting   box   be    used    as   a   speed   controller? 

Ques.  (19)  Why    is   a    motor    generator   necessary    for    a  wireless    set? 

Ques.  (20)  State   several    causes  for   a   generator  failing   to   generate   while   the   motor  is   running. 

Ques.  (21)  What    effect   has    an   increase    of   the    speed    of   a    motor    generator? 


PART  V. 
Storage  Batteries. 

Ques.   (1)     Describe   the    points   of    difference   between    the   lead-plate   storage    cell   and   the   nickel— 
iron-alkali  storage   cell. 

Ques.   (2)     What   is   the   fully   charged   and    discharge   voltage   of   each? 

Ques.   (3)     Explain  the  use  of  a  hydrometer? 

Ques.   (4)     How   often   should   a   storage    cell   be   placed    on   charge? 

Ques.   (5)     What   care   is   necessary   from   day  to   day   to   keep   storage   cells   in   first   class   condition? 

Ques.   (6)      How   can   you  locate    an   open   circuit   in    a   storage   cell    battery? 

Ques.   (7)     What   is   the   normal   charging   rate   of   a   storage   cell? 

Ques.   (8)      What   is  the  normal  discharge  rate   of  a  storage   cell? 

Ques.    (9)      Explain    the    use    of    an    overload    circuit    breaker    in    the    charging    circuit    of    a    storage 
battery? 

Ques.    (10)      Explain    thoroughly    the    use    of    an    underload    circuit    breaker    in    a     storage    battery 
charging   circuit. 

Ques.    (11)      Describe   and   explain  the   use    of  the    amperehour   meter. 

Ques.   (12)     What    causes    sulphating    and    buckling    of    storage    cell    plates    and    how    many    it    be 
prevented? 

Ques.   (13)     State  at  least   two  methods   by  which  the  amount   of   charge  of   a  storage  battery  may 
be    determined? 

Ques.   (14)     What   is    the    essential    difference    between    a    primary    and    a    storage    battery? 
Ques./  (15)     What    will    be    the    effect    if    a    lead    cell    is    discharged    too    rapidly;    overcharged    or 
charged    in   the   wrong   direction? 


PART  VI. 
Condensers,  Oscillation  Generators  and  Radio  Transmitters. 

Ques.   (1)     Define   and   describe   the   construction   of   a  condenser. 
Ques.   (2)     Describe   three   types   of   high-voltage   condensers. 

Ques.   (3)      Explain    the    points    of    difference    in    construction    between    a    high-voltage    and    a    low- 
voltage   condenser. 

Ques.   (4)     Draw  a  diagram  showing  a  number  of  condensers  connected  in  series;   in  parallel. 
Ques.   (5)      If   a    condenser   of    .002    microfarads    and    another    of    .005    microfarads    are    connected    in 
series,   what   is   the   resultant   value   of    capacity? 

Ques.   (6)     What  is   the   capacity   of   a   standard   Marconi   Leyden  jar? 


APPENDIX. 


313 


Ques.    (7)      Describe     fully    what    takes    place    when    a    condenser    discharges    across    a    spark    gap 
through    a    coil    of   wire. 

Ques.    (8)     What   is  meant  by   the   "time   period"   of  an  oscillation  circuit? 

Ques.   (9)     What    are    radio-frequent    electrical    oscillations? 

Ques.   (10)      Explain   the   term   "wave  length"   as  applied  to  an   oscillation   circuit. 

Ques.   (11)     If    the    capacity    of    the    condenser    in    a    closed    oscillation    circuit    be    increased,    what 
effect   has   such   increase  upon   the   wave  length? 

Ques.    (12)     What    effect   has    an   increase    of   inductance   in   an   oscillation    circuit? 

Ques.  (13)  Draw  a  diagram  of  an  open  oscillation  or  radiating  circuit,  and  describe  briefly  the 
use  of  the  apparatus  included  therein. 

Ques.  (14)  How  may  an  aerial  be  turned  to  radiate  a  wave  longer  than  the  natural  or  funda- 
mental wave;  how  can  it  be  tuned  to  radiate  a  wave  shorter  than  the  natural  wave? 

Ques.   (15)      Show  by  diagram  four   methods  for  setting  an   antenna   into   electrical  oscillation. 

Ques.    (16)      Show    by    diagram    the    distinction    between    damped    and    undamped    oscillations. 

Ques.   (17)     What  is  meant   by   the   logarithmic   decrement  of   the   antenna   oscillations? 

Ques.   (18)     What  is  the   purpose   of  a   short  wave   condenser    in  a   transmitting  system? 

Ques.  (19)  How  can  you  tell  if  the  condensers  in  the  closed  oscillation  circuit  of  a  transmitter 
are  punctured? 

Ques.  (20)  Explain  the  changes  necessary  to  reduce  the  wave  radiated  by  a  transmitting  set 
from  600  to  300  meters. 

Ques.  (21)  Draw  a  complete  fundamental  circuit  diagram  of  a  radio  transmitting  set,  and  explain 
briefly  the  function  of  each  part  of  the  equipment. 

Ques.   (22)      How    are    electromagnetic    waves    produced? 

Ques.    (23)      Define    the    term   resonance    as    applied    to   wireless    telegraph    circuits. 

Ques.    (24)      How  are   two   circuits   placed   in   resonance? 

Ques.  (25)  What  is  meant  by  the  term  spark  frequency  and  upon  what  does  the  pitch  or  the 
tone  of  the  spark  depend? 

Ques.  (26)  Define  the  term  damping  and  show  by  diagram  the  apparatus  that  produces  damped 
oscillations. 

Ques.   (27)     Define   a   sharp  wave;    a   pure   wave;    what  is   meant   by  the   term  broad   wave? 

Ques.    (28)      Are    broad    waves    caused   by    highly    damped    or    by   slightly    damped    oscillations? 
Ques.   (29)     How    does    coupling    effect    the    sharpness    of    the    wave? 

Ques.   (30)     What    is    meant    by   the    electrical    length    of    a    conductor    in    radio? 

Ques.    (31)     Why    are    transformers    used    instead    of    induction    coils    in    transmitting    sets? 
^  Ques.    (32)     Which  is  the  most  efficient  transmitting  set,   a  conductively  or  inductively  coupled  set? 

PART  VII. 
Appliances  for  the  Radio  Transmitter. 

Ques.   (1)     Name  and  describe  four  types  of  spark   dischargers  employed  in  radio  telegraphy. 

Ques.   (2)      State   the    advantages   and   disadvantages   of   each   type. 

Ques.  (3)  Suppose  a  secondary  section  of  a  high  voltage  transformer  should  burn  out,  how  would 
you  make  a  temporary  repair? 

Ques.  (4)  What  are  the  essential  points  of  difference  in  the  construction  of  an  open  core  and  a 
closed  core  transformer? 

Ques.   (5)     Give    a    detailed   sketch    showing   the    construction   of   an    open    core    transformer. 

Ques.   (6)     How   would    you    synchronize    a    rotary   spark    gap? 

Ques.   (7)     Describe    the    construction    of    a    quenched    gap. 

Ques.  (8)     Describe   a   primary  reactance   coil   and   explain   its   use. 

Ques.  (9)  Why  is  the  short  wave  condenser  of  a  transmitting  set  composed  of  several  condenser 
jars  connected  in  series? 

Ques.  (10)     Show  how  the  coupling  of  a  transmitting  set  can  be  varied  by  means  of  a  variometer? 


PART  VIII. 

Aerials  or  Antennae. 

Ques.   (1)     Draw  a  diagram  of  a  flat  top  aerial;    an  umbrella  aerial;   a  "T"   aerial;   a  vertical  aerial. 

Ques.  (2)  Which  of  the  four  types  is  the  most  advantageous  for  commercial  wireless  telegraph 
working? 

Ques.    (3)     At  what   point  must  the  antenna  or   aerial  wires  be  well  insulated? 

Ques.   (4)     What  care  is  necessary  to  keep  a   ship's  aerial   in  first  class  shape? 

Ques.   (5)     How   many   wires    are    used    in    the   usual    Marconi    ship    aerial? 

Ques.    (6)      Show  by   diagram  a  test  for  determining  if  the  antenna  insulators  are  leaking. 

Ques.   (7)     How    would    you    temporarily    repair    a    leaking    insulator? 

Ques.   (8)     What    is   the   natural  wave  length   of  the   average   ship's   aerial? 

Ques.   (9)     What   is   the   capacity   in   microfarads   of   the   average   ship's  aerial? 

Ques.   (10)     What    is    the    inductance    in    centimeters    of    the    average    ship's    aerial? 

Ques.  (11)  Can  an  aerial  which  has  a  natural  wave  length  of  600  meters  be  operated  at  the 
wave  length  of  300  meters? 

Ques.   (12)     What  is   the   effect   of  increasing   the  number   of  wires  of  an  antenna? 

Ques.   (13)     How  -may    an    aerial   be    adjusted   to    radiate   a    sharp   wave? 

Ques.  (14)  How  would  you  adjust  an  aerial,  the  natural  wave  length  of  which  was  525  meters, 
to  radiate  a  wave  of  300  meters? 


314  PRACTICAL   WIRELESS   TELEGRAPHY. 

PART  IX. 

Receiving  Apparatus. 

Ques.    (1)  Draw    a    fundamental    circuit    diagram    of    an    inductively    coupled    receiving    transformer? 

Ques.    (2)  Draw  a   diagram   of   the    direct    coupled    receiving   transformer. 

Ques.    (3)  Show   by    diagram   the    circuit    best   suited    to   the    carborundum    detector. 

Ques.   (4)  Draw   a    wiring   diagram   of   a   tuner   suitable   for   the    Marconi    magnetic    detector. 

Ques.  (5)  Draw  a  diagram  showing  the  construction  of  the  Marconi  magnetic  detector,  and  explain 
its  operation. 

Ques.  (6)  Draw  a  circuit  diagram  of  the  tuning  circuits  for  the  Fleming  valve  detector;  also 
for  the  Marconi  multiple  tuner. 

Ques.  (7)  Show   by   diagram  and    explain    fully   how   a    telephone    receiver   operates. 

Ques.  (8)  What   is   the   use   of   a   potentiometer   in    a    receiving   set? 

Ques.  (9)  How  would  you  adjust  a  receiving  tuner  for   ''broad"   tuning? 

Ques.  (10)  How  would   you  adjust  a  receiving  tuner  for    "sharp"   tuning? 

Ques.  (11)  Show  by  diagram  how  you  would  locate  an  open  circuit  in  the  coils  of  a  receiving 
tuner. 

Ques.    (12)      Show  by  diagram  how  you  would   locate   a   short  circuit  in  a  variable   condenser. 
Ques.   (13)     What    is    the    use    of    the    variable    condenser    in    shunt    to    the    secondary    winding    of    a 
receiving   transformer? 

Ques.    (14)     What   is    the    function    of    the    fixed    condenser   in    a   receiving   set? 

Ques.  (15)  How  would  you  adjust  a  carborundum  detector  to  its  maximum  degree  of  sensitive- 
ness? 

Ques.  (16)  Explain   a    buzzer  tester   and  how  it  operates. 

Ques.  (17)  How    could    a    receiving    set    be    calibrated    with    a    wavemeter? 

Ques.  (18)  Explain    the    use    of    an    aerial    changeover    switch. 

Ques.  (19)  How    do    you    protect    the    receiving    apparatus    from   lightning    discharges? 

Ques.  (20)  Describe  a  variable   condenser  and  its   effect   on   the  various   circuits  of  a   receiving   set. 

Ques.  (21)  What   is   the    function   of   a   crystalline    detector   in   receiving    signals? 

Ques.  (22)  Why    are    high    resistance    telephones    used    in    receiving    sets? 

Ques.  (23)  How  do  you  protect  a  crystalline   detector  from  the  local  transmitter? 

Ques.  (24)  Show   by    diagram   a   test   buzzer   and   its  uses. 

Ques.  (25)  Why  should  the  coupling  be  reduced,  when  receiving,  to  the  point  where  the  signals 
are  just  readable? 

Ques.  (26)  Show  how  the  primary  and  secondary  circuits  of  a  receiving  tuner  are  adjustFll  to 
resonance? 

Ques.  (27)  Show  how   the  coupling   may  be   decreased  with   a   conductively   coupled    receiving   set? 

Ques.  (28)  Show    by   diagram   the    simplest    possible    apparatus   for   the    reception   of   signals? 

PART  X. 
Emergency  Apparatus  and  Auxiliary  Transmitters. 

Ques.   (1)     Draw   a   fundamental    diagram   of  the   Marconi   ten-inch   coil   auxiliary   set. 

Ques.  (2)  Draw  a  circuit  diagram  showing  the  charge  and  discharge  circuits  of  a  60  cell  storage 
battery  of  the  type  employed  in  connection  with  the  2  K.  W.  500  cycle  motor  generator. 

Ques.    (3)      When    would    you    make    use    of    the    auxiliary    transmitter? 

Ques.  (4)  What  is  the  maximum  distance  which  an  auxiliary  set  must  be  capable  of  transmitting 
according  to  the  United  States  regulations? 

Ques.  (5)  Draw  a  complete  circuit  diagram  of  the  Electric  Storage  Battery  Company's  charging 
panel. 

PART  XL 
Practical  Radio  Measurements. 

Ques.  (1)  Explain  fully  by  diagram  the  method  of  tuning  a  transmitting  set  to  the  standard 
international  waves. 

Ques.   (2)      Show  by  diagram   a   test  for  determining  the  purity  or  sharpness   of  the  radiated   waves. 

Ques.  (3)  Show  by  diagram  and  explain  the  method  of  tuning  a  transmitting  set  to  resonance  by 
an  aerial  ammeter. 

Ques.  (4)  Show  by  diagram  and  explain  how  a  receiving  set  can  be  tuned  to  a  standard  wave 
length. 

Ques.  (5)  Show  by  diagram  and  explain  the  method  of  measuring  th«  logarithmic  decrement  of 
a  group  of  oscillations. 

Ques.  (6)  Calculate  the  inductance  of  a  coil  of  wire  12  inches  in  length,  3%  inches  in  diameter 
wound  closely  with  No.  28  S.  S.  C.  wire. 

Ques.   (7)     Explain  the  apparatus   for   and  the  method  of  plotting   a  resonance   curve. 

PART  XII. 

Marconi  Transmitters. 

Ques.  (1)  Draw  a  circuit  diagram  showing  the  circuits  of  radio-frequency  for  the  Marconi  2  K. 
W.  500  cycle  panel  set. 

Ques.  (2)  How  would  you  adjust  the  quenched  spark  transmitter  of  this  set  for  a  clear  note  and 
maximum  flow  of  antenna  current? 


APPENDIX.  315 

Ques.  (3)  Explain  the  functioning  of  the  wave-length  changing  switch  in  the  Marconi  2  K.  W. 
500  cycle  set. 

Ques.  (4)  State  the  condenser  capacity  of  the  2  K.  W.  500  cycle  panel  set;  the  ^  K.  W.  500 
cycle  panel  set;  the  1  K.  W.  60  cycle  composite  set. 

Ques.  (5)  How  do  you  change  from  the  quenched  spark  gap  to  the  rotary  gap  in  the  new  panel 
sets? 

Ques.   (6)     How   do   you   adjust   the   acceleration   of   the   starter    arm   on   the   automatic   starter? 

Ques.  (7)  If  upon  closing  the  starting  switch,  the  automatic  starter  failed  to  work,  where  would 
you  look  for  the  trouble? 

Ques.  (8)  Explain  fully  the  steps  necessary  to  tune  the  2  K.  W.  panel  transmitter  for  the  maxi- 
mum flow  of  antenna  current  at  the  three  standard  waves. 

Ques.  (9)     How  do  you   change   the   length  of  the  radiated  wave  on   the   panel  transmitter? 

Ques.  (10)  How  would  you  synchronize  a  rotary  spark  gap  of  a  Marconi  set  and  adjust  it  for 
clear  tones? 

Ques.   (11)     How   may  the    power   input   of   the   panel    sets   be   reduced? 

Ques.  (12)  State  fully  the  steps  necessary  to  disassemble  a  quenched  spark  gap,  clean  it  and  put 
it  together  again. 

PART  XIII. 
Marconi  Direction  Finder. 

Ques.    (1)  Show   the    fundamental    circuits    of   the    Marconi    direction   finder    complete. 

Ques.   (2)  Show    how    the    position    of   a    radio    station    may    be    located. 

Ques.   (3)  How  may  the  direction  finder  circuits  be  preadjusted  to  standard  wave  lengths? 

Ques.   (4)  Explain    how   the    direction    finder   is   tuned   to    a   transmitter. 

PART  XIV. 
Undamped  Oscillation  Generators. 

Ques.   (1)     Show   by   diagram   the   circuits   of  the   arc   generator. 
Ques.   (2)     How   are   continuous   waves  generated? 

Ques.   (3)      Show    by    diagram    the    difference    between    continuous    and    discontinuous    waves. 
Ques.   (4)     Describe   briefly   the   principle   of   the    Goldschmidt   alternator. 

Ques.  (5)  Explain  by  diagram  the  system  for  increasing  the  frequency  of  a  radio-frequency  alter- 
nator, by  means  of  transformers. 

PART  XV. 
Receivers  of  Undamped  Oscillations. 

Ques.   (1)      How  can   undamped    oscillations   be   made   audible   at   the  receiving   station? 
Ques.   (2)      Explain   the    operation   of  the  tikker,    the    heterodyne   and   the  regenerative   beat   receiver, 
Ques.   (3)      Describe    Marconi's    method    for    making    undamped    oscillations    audible    with    a    crystal 
rectifier   detector. 

LOCATION  OF  TROUBLE. 

The  questions  herewith  presented  do  not  bear  directly  upon  the  text  in  this  volume. 
It  is  expected  that  they  will  be  used  by  the  Instructor  in  charge  who  will  explain  the 
various  methods  of  locating  trouble  in  detail. 

Ques.   (1)     If   the   storage   cells   of   a   radio   set  gas   violently  while   charging,   what   is   the   cause? 

Ques.  (2)  If  the  hydrometer  indicates  that  the  specific  gravity  of  a  storage  cell  is  too  low  (when 
the  cell  has  been  fully  charged),  how  would  you  proceed  to  bring  gravity  up  to  normal. 

Ques.  (3)  If  one  cell  of  a  storage  battery  unit  did  not  show  full  voltage,  how  would  you  proceed 
to  bring  it  to  normal  voltage? 

Ques.   (4)     How   would    you    remedy   sparking   at   the   brushes   of   a   motor? 

Ques.   (5)     If   the   motor   refused   to    start,   what  might  be   the  cause   of  the   trouble? 

Ques.  (6)  If  the  release  magnet  of  a  starting  box  burned  out,  how  would  you  keep  the  motor 
in  running  order? 

Ques.  (7)  If  one  or  more  of  the  resistance  coils  of  a  starting  box  burned  out,  how  could  the 
motor  be  started? 

Ques.   (8)      Show  by  diagram  a  method  of  constructing  a  salt  water  rheostat  for   temporary  use. 
Ques.   (9)     Show  how  a   bank   of  lights   may  be  used   for  the  same   purpose? 

Ques.  (10)  How  many  lamps  would  be  required  in  a  circuit  (as  a  lamp  bank  resistance)  to  allow  30 
amperes  to  pass? 

Ques.   (11)     If  a   dynamo  refused  to   generate,   what  might  be   the  trouble? 

Ques.  (12)  Show  by  diagram  a  test  to  determine  trouble  in  either  the  primary  or  secondary  wind- 
ings of  a  transformer? 

Ques.  (13)  If  the  spark  of  a  radio  transmitter  discharges  irregularly,  what  might  be  the  cause  of 
the  trouble? 

Ques.   (14)     What   is   the   effect   if   the  spark   gap   of   a   transmitter   is   opened   too   wide? 

Ques.  (15)  If  an  entire  bank  of  high  voltage  condensers  should  puncture,  how  would  you  manage 
to  maintain  communication? 

Ques.  (16)  What  would  be  the  effect  if  the  synchronous  rotary  spark  gap  accidently  gets  out  of 
synchronism?  . 


316  PRACTICAL   WIRELESS   TELEGRAPHY. 

Ques.  (17)  If  one  of  the  condensers  in  a  transmitting  set  breaks  down,  what  would  you  do  to 
place  it  in  working  order? 

Ques.   (18)      How   may  a   receiving  tuner   be   adjusted  to   decrease   the   effects  of  static? 

Ques.    (19)      How  would  you  test  for  an  open  circuit  in  the  coils  of  a  receiving  tuner? 

Ques.  (20)  If  a  telephone  tests  open  when  the  tips  of  the  cords  are  connected  to  the  terminals  of 
a  battery,  what  might  be  the  trouble  and  how  can  you  remedy  it? 

Ques.  (21)  If  during  the  reception  of  signals  they  weaken  with  the  roll  of  the  ship,  what  might 
be  the  trouble  and  how  can  it  be  remedied? 

Ques.  (22)  If  your  antenna  blew  down  at  sea  so  that  only  a  portion  of  the  wire  remained,  how 
would  you  rig  up  the  transmitting  apparatus  for  communication? 

Ques.  (23)  Show  by  diagram  a  test  for  determining  a  leak  in  the  antenna  circuit.  Show  the 
connections  for  this  test  and  name  one  remedy  for  it. 

INTERNATIONAL  REGULATIONS  AND  U.  S.  LAWS  AND  REGULATIONS 

(Prepared  by  H.  Chadwick.) 

Ques.   (1)     What    is   the    International    Distress   signal? 

Ques.   (2)      What    is    the    distance    requirement    for    auxiliary    apparatus? 

Ques.  (3)  Are  you  compelled  to  exchange  wireless  telegraph  traffic  with  ships  and  Stations 
regardless  of  the  system  used? 

Ques.   (4)      State   the   law   regarding   the   use    of    unnecessary   power. 

Ques.   (5)      What   is   the    rule   in   regard   to   the   disposal   of  traffic   through   the   nearest   coast   station? 

Ques.   (6)      Has  the  sender  of  a  message  the  right  to   designate  the  routing   of  his  message? 

Ques.   (7)     What   are   the   penalties   for   violations   of   the   U.    S.   Radio    Laws? 

Ques.    (8)     What  are   the  international   normal   wave   lengths  for  a  ship   station? 

Ques.   (9)     What   other   wave   lengths   may   be   used   by   ship   stations   and  when? 

Ques.   (10)     What   wave   lengths   are   standard   for   commercial   radio   traffic? 

Ques.    (11)      What    shall    the    charge    for    radiograms    comprise? 

Ques.   (12)      May    operating    practice    be    indulged   in    aboard    ship? 

Ques.    (13)     What  is   the   international   regulation  regarding   the   use   of   superfluous   signals? 

Ques.   (14)     What  is  the   U.   S.  law  in   regard  to  the  secrecy  of  messages? 

Ques.  (15)  What  are  the  U.  S.  requirements  for  an  auxiliary  equipment;  also  the  international 
requirements? 

Ques.   (16)      From   whom   are    charges    for    radiograms    generally   collected?      State    exceptions. 

Ques.   (17)      What   code   of   signals   is  used  for  wireless    telegraph    communication? 

Ques.   (18)     Does   the    coast   or   ship   station   determine   the   order   of  working? 

Ques.   (19)     By  what  method  would  you   call  a  station? 

Ques.   (20)     When  would   you    use   the   "CQ"   call? 

Ques.  (21)  After  calling  a  station  three  times  without  result,  how  long  must  you  wait  before 
repeating  a  call? 

Ques.   (22)     Give   the   following    signals: 

(a)  The  attention  signal. 

(b)  The    termination    of    a    message. 

(c)  The    "go    ahead"    signal. 

Ques.    (23)      What   is   the   rule   in   regard   to    the   transmission   of  long   messages? 

Ques.  (24)  According  to  the  International  Regulations  how  often  may  a  message  be  repeated 
before  being  cancelled? 

Ques.   (25)     What    signal    designates    the    conclusion    of    correspondence? 
Ques.   (26)     Give  as   many   of  the  International  abbreviations  as  possible. 

SECTION  G. 

THE    RELATION    OF  'SPARK   GAP   ACTION  TO    COUPLING  AND   PURITY 

OF  THE  RADIATED  WAVE. 

Frequently  much  confusion  arises  in  the  mind  of  the  radio  student  as  to  the  existence 
and  cause  of  two  waves  being  radiated  by  a  wireless  telegraph  transmitter.  It  should 
be  understood  that  a  transmitter  in  proper  adjustment  for  practical  use  never  radiates 
a  double  wave,  but  in  fact  should  always  radiate  a  single  wave,  of  wave  length  and  damp- 
ing normal  to  the  antenna  circuit  as  adjusted,  but  with  any  transmitter  two  waves  will 
appear  if  the  spark  gap  is  not  in  proper  condition.  The  remedy  in  event  of  the  latter  is 
to  restore  the  gap  to  its  proper  working  qualities  or  to  loosen  the  coupling  at  the  oscilla- 
tion transformer. 

When  the  spark  discharges  across  the  gap  it  acts  as  a  trigger  to  start  the  primary 
circuit  into  oscillation  and  the  stored  energy  of  the  condenser  will  be  transferred  to  the 
antenna  circuit  until  in  the  course  of  a  few  oscillations  (the  number  decreasing  as  the 
coupling  is  closer),  the  voltage  in  the  primary  circuit  becomes  so  low  that  the  spark  will 


APPENDIX.  317 

no  longer  discharge  across  the  gap;  the  primary  oscillations  will  then  cease.  The  exact 
value  of  the  minimum  voltage  for  non-sparking  will  depend  upon  the  resistance  of  the 
gap. 

The  resistance  of  the  spark  gap  always  increases  as  the  oscillating  current  decreases 
and  if  it  were  not  for  the  burnt  gases  which  exist  in  the  immediate  vicinity  of  the  dis- 
charge gap,  the  original  resistance  would  be  restored  at  the  end  of  the  first  half  oscilla- 
tion. Since  there  is  a  lag  in  the  cooling  and  dissipation  of  the  hot  gases,  this  does  not 
occur,  but  if  the  gap  is  properly  cooled,  that  is  the  electrodes  do  not  get  too  hot,  the 
resistance  becomes  so  high  after  a  few  oscillations  that  the  reduced  voltage  of  the  con- 
denser cannot  maintain  the  spark.  In  other  words,  the  spark  is  quenched  and  the  oscilla- 
tions of  the  two  circuits  will  take  the  form  shown  in  Fig.  118,  that  is,  after  a  few  swings 
of  the  primary  circuit,  the  primary  oscillations  will  cease  and  the  antenna  circuit  will 
oscillate  at  its  natural  frequency  and  decrement. 

These  are  the  precise  actions  taking  place  in  a  properly  adjusted  radio  transmitter, 
no  matter  what  type  of  spark  gap  is  employed,  if  the  electrodes  are  clean  and  smooth, 
the  ventilation  adequate  and  the  coupling  of  the  oscillation  transformer  is  not  too  close. 

The  rate  at  which  the  gases  in  the  gap  are  dissipated  or  the  non-conducting  qualities 
of  the  gap  restored,  determines  how  close  the  coupling  can  be  made  without  interfering 
with  the  quenching  of  the  spark.  If  the  coupling  is  so  close  that  the  reaction  of  the 
secondary  upon  the  primary  not  only  does  not  extinguish  the  spark  but  transfers  energy 
back  to  the  primary  then  the  spark  will  not  be  quenched  until  the  energy  of  the  entire 
system  has  fallen  to  a  low  enough  value  to  allow  the  high  resistance  of  the  gap  to  be 
restored.  The  complex  oscillations  shown  in  Fig.  109  will  then  result  and  the  effect  of 
two  waves  radiated  will  be  produced.  If  the  spark  quenches  after  one  or  two  of  the 
"beats"  shown  in  Fig.  109  and  the  antenna  still  has  energy  to  be  radiated,  then  investiga- 
tion with  a  wave  meter  may  show  three  apparently  different  wave  lengths  radiated. 
.  In  all  spark  gaps  there  is  a  tendency  toward  "arcing,"  that  is,  for  the  spark  to  be  fol- 
lowed by  passage  of  the  power  current  across  the  gap.  This  will  prevent  the  restoration 
of  the  high  resistance — the  spark  will  not  quench. 

The  plain  open  spark  gap  without  artificial  means  of  cooling  requires  very  careful 
adjustment  and  reduction  of  coupling  to  give  proper  operation,  that  is  freedom  from 
double  wave  emission.  Unless  the  spark  voltage  is  carefully  adjusted  the  tendency 
toward  arcing  is  difficult  to  control  and  the  action  tends  to  be  irregular.  The  use  of 
special  cooling  means,  such  as  a  series  of  gaps  or  an  air  blast  enables  good  quenching  to 
be  obtained  with  sufficient  regularity  to  give  a  clear  spark  tone. 

The  improved  forms  of  spark  gap,  such  as  the  rotary  and  so-called  "quenched  gap" 
(i.  e.  multiple  plate  gap)  have  the  advantage  of  giving  good  quenching  with  a  tighter 
coupling  and  hence  a  smaller  number  of  oscillations  in  the  primary  circuit.  This,  of 
course,  means  higher  efficiency  and  greater  antenna  current  for  a  given  input  because 
less  energy  is  lost  in  heat  in  the  primary  circuit. 

With  equal  perfection  of  apparatus  there  is  little  to  choose  between  these  gaps  so  far 
as  tightness  of  coupling  without  failure  to  quench  is  concerned,  as  is  evidenced  by  the 
recent  Marconi  sets  which  are  provided  with  both  forms  of  gap. 

If  either  of  these  gaps  gets  into  bad  condition,  or  becomes  overheated,  double  waves 
will  be  radiated,  showing  that  the  spark  has  ceased  to  be  extinguished  after  the  energy 
has  been  transferred  to  the  antenna.  This  can  only  be  remedied  by  restoring  the  gap 
to  normal  condition  or  loosening  the  coupling.  If  the  gap  is  in  particularly  bad  condi- 
tion, loosening  the  coupling  will  not  prove  effective. 

Proper  adjustment  of  the  spark  gap  is  very  difficult  to  obtain  unless  the  power  supply 
circuit  is  properly  designed.  It  may  be  said  that  no  form  of  spark  gap  will  quench  reli- 
ably if  the  source  of  current  supply  is  effectively  short  circuited  by  the  spark.  The 
power  circuit  must  have  sufficient  inductance  or  resistance  so  that  the  voltage  applied  to 
the  condenser  will  fall  when  the  spark  passes,  otherwise  arcing  will  be  produced,  quench- 
ing will  not  take  place  and  a  complex  wave  emission  will  result. 

SECTION  H 

IMPORTANT    REMARKS    ON    THE    DESIGN,    ADJUSTMENT    AND    USE    OF 
1  RECEIVING  TRANSFORMERS. 

According  to  the  grade  of  service  for  which  it  is  employed,  the  receiving  oscillation 
transformer  may  have  either  a  secondary  coil  with  a  fixed  number  of  turns,  without  a  shunted 


318  PRACTICAL   WIRELESS   TELEGRAPHY. 

condenser ;  a  secondary  having  a  variable  number  of  turns,  without  a  shunted  condenser ;  or 
either  type  of  secondary  with  a  shunted  variable  condenser.  In  practically  all  types  of 
transforr-iers,  variable  coupling  between  the  primary  and  secondary  is  provided. 

The  most  efficient  secondary  circuit  is  probably  the  simple  non-variable  coil,  minus  a 
shunt  variable  condenser,  provided  the  coil  is  so  constructed,  that,  with  detector,  stopping 
condenser  and  telephones  connected,  it's  natural  period  of  oscillation  is  that  of  the  signal 
being  received. 

Such  a  circuit  has  often  been  called  an  untuned  circuit,  meaning  that  it  does  not  possess  a 
frequency  varying  condenser  or  inductance.  By  some  this  has  been  assumed  to  mean  a  non- 
oscillatory  circuit  and  the  high  resistance  of  the  detector  has  been  stated  to  be  the  reason 
for  so  calling  it.  The  error  of  this  view  has  often  been  pointed  out  and  Kolster  has  shown 
(proceedings  Institute  Radio  Engineers,  Vol.  1,  page  25)  that  such  a  circuit  gives  a  perfectly 
normal  resonance  curve  of  fairly  small  decrement  and  hence  is  oscillatory  and  resonant  to  a 
particular  frequency. 

If  resonance  curves  of  such  a  circuit  be  taken,  it  will  be  found  that  the  behavior  of 
the  circuit  varies  with  the  coupling  and  with  the  setting  of  the  detector.  For  a  given 
adjustment  of  the  crystal,  the  shape  of  the  resonance  curve,  and  the  location  of  the  maximum 
point  will  vary  with  the  coupling.  The  curve  is  broader  for  close  coupling. 

If  resonance  curves  be  taken  for  a  fixed  loose  coupling,  but  with  a  number  of  different 
crystal  settings,  it  will  be  found  that  the  highest  points  in  the  curves  do  not  coincide,  but 
occur  at  different  wave  lengths  for  different  crystal  adjustments  and  that  the  curves  will 
also  differ  in  shape,  some  being  sharp  and  others  broad. 

The  reasons  why  such  a  circuit  is  oscillatory  is  that  the  coil  has  distributed  capacity 
which  is  equivalent  to  a  condenser  shunted  across  the  inductance.  This  capacity  is  increased 
by  the  capacity  of  the  connected  elements.  The  reason  w,hy  it  is  resonant  is  that  its 
inductance  and  capacity  are  made  such  that  its  natural  time  period  is  the  same  as  that  of  the 
signal  to  be  received.  A  small  shunted  capacity  tends  to  impress  a  high  potential  on  the 
detector  for  a  given  amount  of  received  energy  and  hence  a  loud  signal  is  obtained.  As  its 
capacity  can  be  made  quite  small,  such  a  simple  non-variable  coil  secondary  is  found  to  be  a 
very  efficient  receiver  when  it's  time  period  agrees  with  that  of  the  signal  to  be  received. 

By  varying  the  coupling  and  the  crystal  adjustment,  therefore,  such  a  circuit  can  be  made 
to  tune  broadly  or  fairly  sharply  and  be  put  in  resonance  for  waves  varying  over  a  sufficient 
range  of  wave  length  to  meet  the  needs  of  a  certain  class  of  service. 

For  the  reception  of  signals  over  a  wider  range  of  wave  lengths,  the  use  of  coils  having 
a  variable  number  of  turns  enables  the  natural  period  of  the  secondary  circuit  to  be  adjusted 
to  cover  such  range  by  varying  the  number  of  turns.  A  coil  of  variable  inductance, 
however,  may  be  less  efficient  owing  to  the  possible  consumption  of  energy  due  to  the  flow  of 
oscillations  in  the  unused  turns. 

Where  efficient  reception  and  also  high  selectivity  is  required  over  a  large  range  of  wave 
lengths,  as,  for  example,  from  300  to  3,000  meters,  a  coil  having  a  variable  number  of  turns 
and  a  shunted  variable  condenser  is  employed.  The  loudest  signals  would  usually  be  obtained 
with  such  a  receiver  by  either  disconnecting  the  condenser  or  using  it  at  a  small  value  of 
capacity  and  properly  adjusting  the  coupling,  a  moderately  close  (but  not  the  closest) 
coupling  being  employed.  Where,  however,  there  is  trouble  from  interference,  better  recep- 
tion will  be  had  by  further  loosening  the  coupling,  reducing  the  inductance  and  increasing  the 
capacity  of  the  shunted  condenser.  By  this  means,  with  an  efficient  detector,  signals  of  very 
small  difference  in  wave  lengths  may  be  separated  or  "tuned  out." 

For  best  results,  however,  the  maximum  capacity  of  the  shunted  variable  secondary 
condenser  should  not  be  large  and  it's  minimum  or  "zero"  capacity  should  be  as  small  as 
possible. 

The  effect  of  the  "over-hanging"  or  unused  turns  may  be  minimized  by  disconnecting  such 
turns  from  the  used  portion  and  this  is  the  purpose  and  function  of  the  "end  turn  switches" 
with  which  the  most  complete  present-day  receivers  are  provided,  as  described  in  Chapter  9. 

SECTION  I. 

The  correct  phase  relation  between  the  currents  of  the  main  receiving  aerial  and  the 
balancing  out  aerial  in  Marconi  Duplex  System  is  obtained  by  grounding  the  balancing 
aerial  at  the  end  nearest  to  the  local  transmitter,  the  coupling  coil  (for  the  balancing  aerial) 
being  connected  in  the  circuit  at  a  considerable  distance  from  the  earthed  end. 


INDEX 


Page 

AERIALS 116 

advantages  or  disadvantages,  various   types 

of 121 

Bellini-Tosi   123 

Bradfield  insulator  for 124 

change-over    switch    for 114,  115 

deck   insulator  for 123,  124 

directional   121 

"dummy"    127 

effective  inductance  and  capacity,  measure- 
ment   of 214 

flat    top 119 

for    direction    finder 256 

function  of 116 

function    of   receiving 129,  130 

fundamental   considerations,   in   design   of..  118 

fundamental   wave  length   of  "T" 119,  120 

induction  of  current  in  direction  finder....  258 

installation   of   ship's 124 

inverted    "L"    type 119 

loading   inductance    for 117 

standard    Marconi   ship 123 

table  of  wave  lengths  of 121 

"T"  type 119 

tuning   elements    for    receiving 130 

tuning  inductance   for 109 

umbrella 119 

vertical 118 

wave    length    of 117 

Alexanderson   Alternator 268 

Alternating   Current 

effective  value  of i 41 

lag  and  lead  of ." 40 

Alternating    Current    Dynamo 26 

Alternator 22,   23 

Alexanderson,    high    frequency 268 

diagram  of,  fundamental 25 

Goldschmidt   radio   frequency 270,  271 

Ammeter   43 

hot  wire 45 

thermo    46 

with  shunt 43 

Ampere   hour 

meter   for    measurement    of 72 

unit  as  applied   to  storage   cells 70 

Amplifier 

microphonic   169,  170 

radio-frequency    163,   164,  284 

telephone 167,  168 

vacuum  valve 163 

Antenna 

decrement  of 126 

definition   of 87 

Arc  Transmitters 265,  266 

Navy    type 266 

signalling  with 267 

Armatures 

drum 31 

dynamo    31 

motor    31,   32 

ring 31 

Armature    Windings 31 

construction    of 56 

development    of 31,  32 

lap 32 

wave 33 

ring    3.1 

Atmospheric    Electricity 171 

Audibility  Meter 217 

Auxiliary    Transmitters 179 

adjustment  of 180,  181 

Marconi   type 179 

requirements  of 179 

tuned    coil    type 180,  181 


BATTERIES 


primary    5 

secondary  6 

Breaker,    Circuit 

underload     72 

Buzzer 

excitation    systems 167 

for   adjusting  detector 167 

tuned   tester    for  direction   finder 258 


Page 

CAPACITY 

at   radio   frequencies,  measurement  of.  .208,  209 

distributed,    effects    of 164 

electrostatic   36,  37 

equation   for 38 

formula   for   standard   of 208 

measurement  of 207,  208 

Carborundum    Detector 

action   of 137 

adjustment  of 139,   140 

theory  of   action 137,    138,    139 

tuning   circuits    for 135 

Care   of   Motor    Generator 65 

Cargo  Transmitting  Set,   Marconi    J/4    K.   W....    248 

Cells 

primary 5,   6 

secondary 6,   7 

grouping  of 9 

in  series 9,    10 

in    parallel 10,    11 

in    series    parallel 11,    12 

internal    resistance    of 10 

Charging   Panels 

Electric  Storage  Battery  Company .  181,   182,   185 

General   instructions    for 187 

Marconi    auxiliary 180,    182 

Choking   Coils 

high    frequency 110,    111 

Circuits 

branch    13 

closed    oscillatory 84 

divided    13 

examples  of 13 

open   oscillatory 84 

reaction   of  coupled 95 

receiving   radio 129 

Closed    Circuit   Transmitter 

calibration  of 200 

Coils 

calculation   of   inductance   of 209,   211,   213 

choking 110,    111 

measurement    of    effective    inductance    of ...    209 
measurement    natural    oscillating    period    of 

tuning 207 

induction    46 

interrunter    for    induction 48 

Compass,  Wireless 255 

Condensers 

battery    of 82 

capacity  of 37 

complete   cycle    of   events    in    the   discharge 

of 83 

connections   for 81 

•  high   potential 81,   110 

how  to  charge 82 

low   potential 81 

motor   generators,    protective   type   for 65 

short    wave If 9 

simple     37 

Conductors    4 

partial 4 

Continuous    Waves 

receivers    for 277 

transmitters  for 2^4 

Converter,    Rotary 57,   58 

Coupling 

capacitive     °T 

coefficient  of 9" 

conductive 94 

direct    excitation 93 

electrostatic    95 

inductive     93,  94 

"tight"    or    "close" .' 96 

"tight"  and   "loose" 219,  2 '0 

transmitter,  determination  of 195 

Currents 

alternation  of 21 

by   chemical    battery 5,   6 

by    dynamo 22 

classification    of 4 

electric     4 

induced     19 

of  audio    frequency,   definition   of 80 

of   radio   frequency,    definition    of 80,  81 

strength   and   quantity   of 7 

thermo-electric 46 


319 


320 


PRACTICAL  WIRELESS  TELEGRAPHY. 


Page 

DAMPING  OF  OSCILLATIONS 90 

calculation  of 93 

measurement      of      logarithmic      decrement 

of 200,  201,  202 

theory  of 83,  90,  91,  92,  93 

Decrement 

of    antenna 126 

calculation  of 127 

Decremeter 

calculation    of   the   decrement   of 203 

Kolster    204 

Detectors,   Oscillation 

carborundum 135 

classification    of 140,  141 

electrolytic    158 

holders   for 140 

magnetic,    Marconi 148,   149,   150 

primary   cell 159 

three  element  valve 159 

valve,  Fleming 141 

zincite-bornite     140 

Diagrams,    Wiring 

Alexanderson    alternator 206 

arc    transmitters 265 

auxiliary   charging   panel 184,   185 

beat  receiver 281 

direction    finder 259 

earthing  system   high  power  stations 305 

Glace  Bay  transmitter 291 

Goldschmidt    alternator 270 

heterodyne    receiver    279 

Joly-Arco   alternator 272 

lay  out  aerials  high   power  stations.  ..  .298,  299 

Marconi    duplex   scheme 289 

Marconi   undamped    receiver 286 

Marconi   l/2    K.   W.    120  cycle  transmitter..    243 
Marconi   2    K.    W.    500   cycle    transmitter.  .    224 
Marconi      l/2      K.     W.      500     cycle     trans- 
mitter     235,  236 

Marconi    Y4    K.    W.   500  cvcle   transmitter..    251 

Marconi    1    K.   W.    60   cycle   transmitter 241 

measurement  of  decrement 196 

Navy   "beat"   receiver 283 

pliotron  oscillator 276 

radio-frequency    amplifier 284 

standard   receiving  tuners.  ...  134,   152,   155,   149 

standard    transmitter 98 

tuned   coil 181 

tuning  of  transmitters 190,   191,   192,   193 

Direction     Finder 255 

aerials  for 256 

circuit  of 257,  258 

direction   of   forces    within 260 

general   instructions   for   operation 260,  261 

Marconi,  description  of 255 

tuned    buzzer   tester   for 258 

to  find  direction  of  radio  station 262 

Dynamo 22 

alternating  current 26 

compound     27,  28 

determination   of    frequency   of 24 

direct    current 27 

magnetic    field    of 24 

principle   of 22,  23,  24 

principle  parts  of 22 

shunt,   series   and   compound 27 

simple   type    of 22,  23 

strength    of  field  of 24 

Dynamotor    57 

EARTH  CONNECTION 124,   125 

at  trans-oceanic  stations 125 

Edison   Storage  Battery 79 

charge    and    discharge   of 79 

Electric  Current 14 

Electric   Meters 42 

Electric     Motor 29 

Electric    Waves 87,  88 

determination  of  length  of 89 

length  of 88 

standard,  for  ships  aerials 96 

wireless    88 

velocity    of 88 

Electrical    Circuits 48,  49 

examples    of 49 

Electrical  Cells,  Grouping  of 8 

Electrical  Devices,  Current  Output  and  Voltage 

Output  of 15 

Electrical  Horsepower 14 

Electrical    Resistance 8 


Page 

Electrical    Resonance 85 

importance    of 188 

indicators     of 190 

Electrical  Units 14 

practical    definition    of 14,    15 

Electrical    Work 13,   14 

Electricity 

atmospheric     171 

frictional    5 

static 5 

Electrification     5 

positive    5 

negative 5 

Electrodes   6 

Electrolyte  6 

Electromagnetism 16 

Electromotive  Force 4 

by  chemical  action 5 

by  dynamo 22 

by   friction -. 5 

by  thermo  couple 47 

effect  of  counter 30 

induced  value  of 22 

production  of 5 

unit  for 5 

Electrostatic  Capacity 37 

End    Turns,    receiving    tuner 164 

switch  for 165 

FREQUENCY 

determination   of 24 

Dynamo 24 

Meter   44 

GALVANOMETER     • 

Construction  of 42 

Generator 

arc,  for  undamped  oscillations 265 

compound 28 

how  to  determine  polarity  of 76 

motor    51,  52,  53,  54,  55,  56 

series    28 

shunt    27 

Glace    Bay — Clifden    Transoceanic    Stations....  291 

Goldschmidt    Radio-Frequency    Alternator.  .270,  271 

17 

.  278 


HELIX   , 

Heterodyne    Receiving   System , 

High    Power    Stations 

condensed    list   of 306 

Marconi     transoceanic 292 

of  the  United   States 305 

Horsepower 

electrical    14 

mechanical 14 

Hydrometer 69 

INDUCTANCE 

calculation     of 209,  210,  211,   212,  213 

correction   factor   for  calculation   of.. ..211,  213 

measurement   of  effective 209 

measurement  of  mutual 216 

of  an   aerial 214,   117,  121 

unit  of 21 

variation   of  a   radio-frequency 165 

variometer 166 

Induction 

electromagnetic    19 

magnetic    1 

mutual    19 

self    20 

1  nsulators    4 

Bradfield    124 

deck 123,  124 

fOf.V       ARCO       STEP-UP       FREQUENCY 

"    TRANSFORMER   .  273 


KEYS 

transmittin 


transmitting    

Key   Control,  G.   E.   Co.'s  type, 


115 
268 


MAGNET 

permanent 

temporary    2 

Magnetism 

natural 

electro    3 


INDEX. 


321 


Page 

Magnetic 

circuit 3 

flux 1 

induction    1 

permeability   2 

polarity    1 

poles,   laws   of 3 

retentivity   2 

Magnetic    Field 3 

about  two   parallel  conductors 17 

strength    of 17,   18 

Marconi 

balancing  out  aerials 291 

continuous  wave  receiver 286 

continuous    wave   transmitter 274,  275 

directional  aerial 292 

duplex  radio    system 290 

transoceanic  radio  telegraphy 288 

transoceanic   stations 292 

tubular    mast 299 

Measurement 

of  coupling 195 

of  decrement 200,  201 

of  electrostatic  capacity 207,  208 

of  high  voltages 220 

of  inductance  of  a  coil 209 

of  oscillating  period  of  a  coil 207 

of  receiving  transformer 205,  206 

of  strength  of  incoming  signals 216,  217 

of   wave   length 191,   192,  193 

Meters 

ammeter 43 

audibility    meter 217 

frequency    meter 44 

galvanometer    42 

hot  wire  meters ,45,  46 

radio   frequency  meters 47 

voltmeter 43 

wavemeter    188,  189 

wattmeter 43,  44 

Motor,    Electric 29 

counter  E.   M.   F.  of 30 

field  of 30 

principle  of 29 

reversal    of 29 

torque  of 30 

with  differential  field  winding 30 

Motor    Generators 51,  52,  53,  54,  55,  56 

care  of 65 

complete  circuit  of,  with  hand  starter 63 

compound  generator  type  of 53 

construction  of 51 

Crocker- Wheeler  type  of 54,  55,  56 

differential   motor   type   of 54 

how   to    remove   armature   of  2    K.   W.    500 

cycle    Marconi 66 

how  to   remove  armature  of    %    K.  W.   500 

cycle  Marconi 240,  241 

regulation  of 53 

shunt  wound 32,  53 

Motor    Starter 58 

automatic    %    K.    W.    Marconi    type    of.. 60,  61 

Cutler-Hammer   type    of 59 

General  Electric  type  of 60 

2  K.   W.  automatic 61,  62,  63,  64,  187 

2   step    y2    K.    W.   automatic 61 

OHMS  LAW 11 

examples  oi 11,   12 

Open  Circuit  Transmitter 

calibration  of .- 199 

Oscillations 

damping  of 83 

determination   of  frequency    of 84 

effect  of  resistance  on 84 

frequency  of 84 

generator  of 84 

highly  damped  group  of 90 

logarithmic   decrement   of   Electrical 90 

methods  of  exciting,  in   an  aerial.  ..  .93,  94,  9! 

of    feeble    damping 90 

variation  of  frequency  of 85 

Oscillation    Constant 86 

Oscillation   Transformer 

adjustment    of    coupling   of 107 

inductive     type 107,  108 

pancake  type 108 

square   coil    type iuv 

variometer  type 108 

Oscillator 

open   circuit 86 

Pliotron   276 


Page 

PANEL       TRANSMITTERS,       AMERICAN 

MARCONI  TYPE 223 

general   instructions    for 253 

Phase    Displacement   of   alternating   currents.  .  .  41 
Polarity 

Magnetic   1 

Power   Circuits 

examples  of 48,  49 

Power    Factor 41 

Protective   Condensers    for  Motor   Generator.  .  .  64 

Protective  Resistance  Rods  for  Motor  Generator  64 

RADIATION  OF   ELECTRIC  WAVES... 87,  125 

Radio-frequency    current,    generation   of 80,  81 

Radiogoniometer 255,  257 

Radio  Measurements,  Practical 189 

Radio    Transmitter 79 

appliances  for 101,  115 

explanation   of 99,  100 

fundamental    circuit   of 97 

numerical   values   for 100 

spark  dischargers  for 

101,   102,   103,   104,   105,  106 

Marconi  panel  types 223 

Range 

calculation   of   transmission 127 

Reactance 

capacity   38 

inductance    39 

regulators    114 

Receiver 

balanced  crystal 172 

capacitively  coupled 134 

directly   coupled 133,  134 

electrostatically  coupled 134 

impedance    of    telephone 131 

inductively  coupled 132,  133 

simple  radio 132 

telephone   magneto 131 

Receiving  Sets 

long   distance 306 

Marconi    balanced    crystal 172 

Marconi-Fleming  valve 141 

Marconi  multiple  tuner 148 

Marconi  type  101 154 

Marconi  type   106 151 

Marconi   type    107-a 144 

Marconi    Universal 157 

Rectifier 131 

Relays 

Brown    170 

microphonic    169 

Resistance,    Electrical 8 

examples    of 9 

radiation 126 

relative,    of    metals 9 

specific 8 

standard  of 9 

unit   of 9 

Resonance   Curves,   Plotting  of...  196,   197,   198,  199 

Resonance,   Electrical 40,  85 

indicators  of,  for  wavemeters. 191 

resonance   in  complete   radio  systems 135 

Rheostat  Field 26 

construction  of 57 

Rotary    Converter 51 

SELF   INDUCTION 

coefficient    of 21 

unit  of 21 

Signals.   Radio 

audibility  meter  for  strength  of 217 

chart  showing  variation  of :  .  218 

measurement  of  strength   of  incoming.  .216,  217 

Solenoid 17 

Spark    Discharge 

analysis    of 83 

Spark   Dischargers 

adjustment    of   note    of 106 

adjustment   of  quenched 247 

details   of  quenched 230 

Non-synchronous     102,  103 

plain   -. 101 

quenched    105 

synchronous    103,  104 

synchronous,   240  cycle 248 

Static  Electricity 5,  171 

Starters.  Motor.  ..                                                 58 


322 


PRACTICAL   WIRELESS   TELEGRAPHY. 


Page 

Storage  Batteries 67 

ammeter  and   underload  circuit  breaker  for 

charging  circuit  of '.  . .  .  72 

acid  spray  of 77 

capacity   of . 69 

charging  process  ot oo 

charging   when   voltage   of   charging   source 

is  less  than  that  of . 74 

determination    of    state   of   charge    and    dis- 
charge of 76 

Edison   type    of 79 

electrolyte  of 69 

Exide   type   of,    general    instructions 

fundamental    actions    of 68,  69 

fundamental   facts   of . 

general  construction  and  action  of 68 

how  to  determine  charging  resistance 71 

lamp  bank   resistance  for 72 

lead    sulphating    of •  •  •  69 

level  of  electrolyte  of 77 

necessity  for 67 

overcharge  of 74 

portable    chloride    type   of,   general   instruc- 
tions    77 

specific  gravity  of •  •  69 

use  of  ampere  hour  meter  for 72,  73 

Switch 

aerial  change-over 1 14 

end  turn •  •  •  165 

type  I,  aerial,   Marconi 173,  174 

TELEPHONES 

Baldwin 168, 

receiving   

response  of 

Tikker    Detector 

Tone  Wheel  Detector 

Transformers,  A.    C 

closed  core 

high  voltage 

magnetic  circuit  of 

magnetic  leakage  gap  for 

open  core " 35, 

oscillation    

radio-frequency 

receiving  radio ii 

step-down 34 

step-up 

Transmitters    

aerial  current  of 

auxiliary    

Marconi  panel  type  of .......  ....  ....  .  .  .  .  . 

Marconi    type    Jr-4 ZZb,  z/o,  zz/,  zzo, 

Marconi  type  P-5 

234,  235,  236,  237,  238,  239, 

radio 

reduction   of  power  of 

type   E-2,   details   and   circuits  of 244, 

type   E-2,   instructions   for   tuning  of 

type    E-2,    Marconi 

type   P-4,   tuning   of 231,  232,  233, 

type  P-4,  adjustment  of 229,  230, 

type    P-5,   description   of 

234,  235,  236,  237,  238, 

type  P-5,  Marconi  adjustment  of 239, 

type    P-9,    description 

type  P-9,   Marconi    ^4    K.  W 

type  P-9,  tuning  of 

]    K.    W.   60  cycle  non-synchronous 

1  K.  W.  60  cycle,  description  of 

1    K.   W.  adjustment  of 

1   K.  W.   installation   of 

-Tuned   Coil  Emergency  Transmitter 180, 

Tuners,    Receiving 

balanced    crystal ............. 

calibration    open    and    closed    circuits    sim- 
ultaneously      J  •  '  '• one 

calibration  of  secondary  and  primary .  .*JUo, 

end    turns   of : 

English   Marconi  Universal.. • ••• 

general    advice   for  manipulation  of....  1/6, 
inductively    coupled,    adjustment    of 


169 
167 
131 

278 

284 

33 

112 

35 

113 

112 

107 

36 

49 
34 


179 
229 


80 
253 
245 
246 
244 
234 
231 

239 
240 
249 
248 
252 
240 
241 
242 
242 
181 
J29 

207 


Page 

Tuners,   Receiving — Continued. 

Marconi    multiple '. 148,   149,   150 

Marconi    type    101 154,   155,   156 

Marconi   type    106 151,   152,   153,  154 

Marconi    type    107-a 144,   145,   146,   147 

selectivity     in 136,   137 

theory   of  adjustment   of 136,   137 

Marconi    type    112 174,   175,   176 

Tuning    188 

aerial    inductance    for 109,   110 

by   hot   wire  ammeter 194 

continuously  variable   inductance   for 109 

general   instructions   for    radio   transmitter.    191 

measurement   of   the   closed   circuit 193 

measurement   of   the   open   circuit 192 

measurement   of  radiated   wave 193,   194 

of    the    2    K.     W.     500    cycle    panel    trans- 
mitter    194,  195 

Tuning  Records 

Marconi 220 

U.   S.   Government 221 

UNDAMPED   OSCILLATIONS 

Alexanderson    alternator  >for 268 

arc    generator    of 265 

beat   receiver   for 280,  281 

Goldschmidt  alternator   for 270,  271 

heterodyne    system    for 278,  279 

Joly-Arco   system   for 273,  274 

Marconi's  system   for 274,  275 

Marconi's    receivers    for 286 

methods  of  generating 264,  265 

Navy    beat    receiver    for 283 

receivers    for 277 

tikker   receiver  for 278 

tone  wheel  for 284 

transmitters    of 264 

Units,    Electrical 14,   15 

of    current 7,  8 

of   electromotive    force 7,   14 

of    power 14,   15 

prefixes  of 15 

of    quantity 8,   1' 

of  resistance 9,  14 

VALVE,   OSCILLATION 

adjustment  of 161 

amplifier   circuits   for 163 

as    combined    detector,    amplifier    and    beat 

receiver     •.•••.•  -280,  281 

as   source   of   radio-frequency   oscillations.  .  280 

audio- frequency  repeater   for 161 

circuits    for    Fleming 143 

Fleming    141,  142 

operation   of    3    element   valve 160 

radio-frequency    amplifier 163 

regeneiative  circuits  for 162 

repeater  circuit   for 161 

three    element 159 

U.   S.  Navy  circuits  for 283 

Variometer 

for    receiver 166 

for  transmitter 108 

Voltmeter    42 

WATTMETER    43,  44 

Wavemeter 

as   a  source  of  high-frequency  oscillations.   205 

calibration    of 214,  215 

measurement  of  natural  wave  length  by  190,   191 

resonant  adjustment  of 190,   191 

uses    of 190 

Waves 

damped 8} 

distortion  ot 1^4 

method   of  changing  length  of 97 

Naval    standard   length   of •  •  •    97 

standard    commercial    wireless o< 

sustained,    apparatus    for.... 265,  266,  267,  268 
undamped    ° 


363013 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


